We had a checkout test at my last job that everyone called "the coin flip." Green for a week, red twice on a Tuesday, green again. Someone eventually wrapped it in a retry and it sat like that for eight months before anyone looked at it again. Turned out the real bug was a webhook that occasionally fired before the order record finished writing to the DB - a two-hundred-millisecond gap that only showed up under load. The test wasn't broken. It was the only thing in the entire pipeline that noticed.
That's usually the story. Someone blames the test - bad selector, missing wait, a sleep(2) some intern left in there three years ago, and half the time they're right. But when a test flakes repeatedly and nobody can explain why, the test is rarely the actual problem. It's just the part of the system rude enough to say something.
A few places I keep finding the real cause hiding.
Tests that quietly depend on each other
Test A writes a row, Test B reads it and never knew it needed to. Run B by itself, it passes. Run the suite in a different order, or in parallel, and B fails for no reason anyone can point to. I've lost a full afternoon to this exact thing more than once - a cache value from Test 12 leaking into Test 47.
