Coverage says 94%. Every test is green. You deploy on Friday afternoon, and by Monday a customer has found a bug in the exact function you "tested". When you open the test, the reason is obvious in hindsight. The test never ran your code. It ran a f...

A user fills in your form and clicks "Create post". The post is saved in the database. And the page shows "Could not create post". They try again. Now there are two posts. Nothing is wrong with your database. Your error handling caught something it...

Here is a pattern I see in almost every React codebase I read: const fullName, setFullName = useState''; useEffect => { setFullNamefirstName + ' ' + lastName; }, firstName, lastName; It works. It also renders twice every time a name changes, sh...

You notice it during a code review: .env is in the repository. Your stomach drops. You run git rm .env, commit with the message "remove env file", push, and feel the relief of a problem solved. It is not solved. The file is gone from your latest com...

You upgrade from Next.js 14 to 15. The app compiles, the tests pass, nothing visibly breaks. A week later your database bill is higher, your pages feel slower, and your Time to First Byte graph has a step in it that lines up exactly with the deploy. ...

A user scrolls through your feed. Page 1 ends with "Post 20". They tap "Next", and page 2 starts with... "Post 20" again. Another user pages through an admin table and never sees order #4817. It exists. It is in the database. It just never showed up...

A customer clicks "Pay now". Nothing happens for three seconds, so they click again. Or their phone loses signal for a moment and the app quietly retries the request. Or your load balancer times out and sends the request again. Your server received ...

You need a theme provider, so you create a file, add 'use client' at the top, and wrap your app in it. Then a nagging thought arrives: "I just wrapped my entire layout in a client component. Did I turn the whole app into client-side React?" Most de...

Your stomach drops. You typed git reset --hard, or deleted the wrong branch, or a rebase ended with half your commits missing. The log looks empty. Three days of work, gone. Take a breath. If you ever committed that work, it is almost certainly stil...

You build a dashboard. Sidebar on the left, content on the right, a table or a code block inside. It looks perfect with your sample data. Then a user pastes a long URL into a field, and the whole page grows a horizontal scrollbar. The sidebar gets s...

You format a date in a component. It looks right on your laptop. Then a user in another country opens the page and either sees a console full of hydration errors, or worse, sees nothing wrong at all and reads the wrong time. I live in a timezone tha...

Your endpoint sends 50 emails, marks 50 orders as shipped, and returns { success: true }. In your tests, it works. In production, the response comes back in 8 milliseconds, half the orders are still pending, and one failed email crashed the whole No...

TypeScript's most dangerous keyword is two letters long. as does not convert anything. It does not check anything. It tells the compiler "trust me", and the compiler does. If you are wrong, the error moves from your editor to your users. Here are f...

You add a small search box to your navbar. It reads the query string, so you reach for useSearchParams. Everything works in next dev. You push, the deploy fails, and the build log says: useSearchParams should be wrapped in a suspense boundary at pag...

Here is a bug report you will eventually get: "The signup form told me what was wrong on your machine, but on the live site it just says something went wrong." Nothing is broken. Next.js is doing this on purpose. But the code that triggers it looks ...

How Much Does a SaaS MVP Cost? Your SaaS MVP cost is decided mostly by four things: how many features you launch with, whether people need accounts to log in, whether you take payments, and how polished the design has to be. A focused MVP built aro...

Almost every Tailwind developer hits this exact bug once, usually right after a deploy, and the confusing part is that nothing about it looks wrong in the code, nothing throws an error, and it worked completely fine five minutes earlier on the same m...

This is the kind of bug that goes from "works perfectly in every test" to "actively leaking one customer's data to another customer" without a single code change in between, purely because of how caching and personalization interact in a way that isn...

This is one of the more consequential Mongoose gotchas because of exactly what tends to live inside a pre'save' hook, password hashing, slug generation, validation logic, and exactly which methods silently bypass it without ever telling you. The Set...

Redirecting a user back to the page they were trying to reach after they log in is good, expected UX. Nobody wants to log in and land back on the homepage when they were actually trying to read a specific article or check out a specific product. The ...