How to Find Bugs on Your Website
Most website bugs are invisible to the people who built the site. You know where everything is, you test on a fast connection, and you probably don't use the device or browser that's causing the problem. This guide covers all four categories of website bug — visual, functional, performance, and SEO — with the right tool and approach for finding each one.
The four categories of website bug
"Website bugs" is a broad term. Before you can find them, it helps to know which type you're looking for — because the tools and methods are completely different.
There are four categories: visual and layout bugs (things that look broken), functional bugs (things that don't work), performance bugs (things that load slowly), and SEO bugs (things that hurt your search visibility). Most sites have problems in all four.
1. Visual & layout bugs
Things that look broken to a customer — regardless of whether the underlying code is "working." This is the most common category and the one most likely to be invisible to developers who test on their own powerful machines.
Common examples
- Broken images (image URL returns 404, CORS blocked, or wrong dimensions)
- Text overflowing its container on smaller screens
- Navigation menu collapsing incorrectly on tablet widths
- Hero section background not loading
- Buttons or links cut off or overlapping other elements
- Dark mode / light mode mismatch causing invisible text
Tool to use: BugSense — renders pages like a real browser at multiple viewport sizes and detects broken images, overflow, and layout failures.
2. Functional bugs
Features that should work but don't. These directly cost you conversions because they block customers from completing something.
Common examples
- Contact form submits but no email is received
- Add to cart button unresponsive on mobile
- Checkout flow failing on a specific payment method
- Password reset link not arriving
- Search returning no results due to a config error
- Cookie banner blocking the page behind it
Tool to use: Manual testing is still the most reliable here. Automated scanners catch some functional failures (like forms that return HTTP errors), but you should test every conversion-critical flow yourself on mobile and desktop.
3. Performance bugs
Pages that load slowly lose customers. A 1-second delay in mobile page load can reduce conversions by up to 20% (Google/Deloitte, 2018). With ~60% of global web traffic now on mobile (Statcounter, 2025), performance problems disproportionately affect the majority of your visitors. Performance issues are usually invisible until you measure them.
Common examples
- Images served at 4000×3000px when displayed at 400×300px
- Render-blocking JavaScript delaying first content paint
- No caching headers on static assets
- Fonts loading late, causing layout shift
- Third-party scripts (chat, analytics) slowing page load
- Large CSS files not split by route
Tool to use: Google PageSpeed Insights (free) and GTmetrix (free tier) both measure Core Web Vitals and identify the specific resources causing slowness.
4. SEO & crawlability bugs
Issues that prevent search engines from indexing your pages correctly. These don't affect current customers, but they quietly erode future organic traffic.
Common examples
- Broken internal links returning 404
- Redirect chains (A → B → C instead of A → C)
- Pages accidentally blocked by robots.txt
- Duplicate title tags across multiple pages
- Missing canonical tags causing index duplication
- Structured data markup errors
Tool to use: Screaming Frog SEO Spider (free up to 500 URLs) is the standard tool for crawl-level SEO bugs. Google Search Console shows which pages Google couldn't index and why.
Start with an automated scan
Manual testing catches what you know to test. Automated scanning catches what you don't. The practical approach is to run automated tools first — they surface the bulk of obvious issues in minutes — then do targeted manual testing on the specific flows that matter most.
For visual bugs: run BugSense first. It renders your pages at desktop, tablet, and mobile viewport sizes and checks for broken images, layout failures, form errors, and other customer-facing issues. No account needed for a free scan.
For performance bugs: run Google PageSpeed Insights on your most important pages (homepage, product/pricing page, contact page). Look specifically at the mobile score — desktop scores are almost always higher.
For SEO and broken links: run Screaming Frog on your full site. Filter for 4xx and 5xx response codes. These are your broken pages.
Manual testing checklist
Automated tools miss anything that requires human judgment or flows through authenticated states. After running your scans, manually test these:
Mobile on a real device
Test your full purchase or conversion flow on an actual mobile device — not just browser DevTools. Tap targets, keyboard behavior, and scroll feel all differ from emulation. Use an iPhone and an Android if you can.
Every contact form and signup form
Submit each form with valid data and check that you receive the notification. Check that validation errors appear correctly for bad input. Broken forms are the single most common source of lost leads.
All images above the fold
Scan each page and look for any image that's loading as a gray box, broken icon, or wrong dimensions. Pay special attention to hero images and product images — these have the highest conversion impact.
Navigation on tablet width (768px)
Tablet is the most neglected breakpoint. Resize your browser to 768px and check that navigation is usable, content isn't cut off, and no elements overlap.
Checkout or signup in a private window
Open an incognito/private window so you're logged out with no cached state. Walk through the full conversion flow. Many bugs only appear for first-time visitors.
The bugs most sites miss
In scans of real sites, the same bugs come up repeatedly — not because developers don't care, but because they're genuinely hard to notice without the right tools:
- Images loaded from the wrong environment. An image works fine in staging because it exists there, but 404s in production because the file was never uploaded. This is very common after migrations.
- Mobile nav that traps the user.A hamburger menu that opens but can't be closed, or closes but leaves an invisible overlay blocking taps. Nearly impossible to catch on desktop.
- Forms that appear to submit but silently fail.The success message shows, but the form data goes nowhere — usually due to an API key change or a CORS issue that wasn't caught after a deployment.
- CSS that breaks in Safari. Safari has stricter handling of certain flexbox, grid, and CSS variable patterns. A layout that looks fine in Chrome can completely break in Safari — which is a significant share of mobile users.
- Pages blocked by a cookie banner. In one scan, we found a store where the GDPR cookie banner was rendering on top of the shopping cart with a z-index high enough to block all taps. The store owner had no idea.
How often should you scan?
Static sites that change rarely can get away with scanning after each deployment. Dynamic sites — e-commerce stores, SaaS apps, marketing sites with frequent content updates — should scan continuously.
The most common time bugs appear is right after a deployment or a third-party update (a theme update, a plugin update, a CDN change). Bugs introduced by these changes can persist for days or weeks before someone notices if you're not monitoring.
BugSense's paid plans include 24/7 uptime monitoring and scheduled scans, so you're alerted when something breaks rather than finding out from a customer complaint.
Frequently asked questions
What is the fastest way to find bugs on a website?
Run an automated scan first. BugSense finds visual and functional bugs in minutes. Google PageSpeed Insights finds performance issues. Screaming Frog finds broken links and SEO issues. Automated tools surface the bulk of bugs far faster than manual testing alone.
How do I check my website for broken links?
Screaming Frog SEO Spider is the most thorough free tool — it crawls your entire site and flags every URL returning a 4xx or 5xx error. For smaller sites, Google Search Console's Coverage report also surfaces crawl errors including broken links.
How do I find mobile bugs on my website?
The most reliable method is to test on real devices. For automated mobile testing, BugSense renders pages at mobile viewport sizes and detects broken layouts, overflowing content, and tap targets that are too small. Chrome DevTools' device emulation is also useful for quick checks.
Can I find website bugs for free?
Yes. BugSense offers a free scan with no account required. Google PageSpeed Insights is free. Screaming Frog is free for up to 500 URLs. Nibbler is free. These tools together cover visual bugs, performance issues, broken links, and basic SEO checks at no cost.