It compiles. It passed review. It fails on the wrong interleaving.
Five levels. Every snippet is generated fresh. Each one is already in production, and each one is broken.
Every case here compiles, passes review, and works on a developer laptop. They fail under load, on the wrong interleaving, on the wrong architecture. Find the line, then name the fix. Clear a level to face a harder one; miss it and you get a new case at the same level, never the one you just saw.
LV.1 · DATA RACESand why volatile is not the answer
LV.5 · SUBTLE BUGShidden ordering, ABA, red herrings
Read-only, deliberately: this page cannot compile or run C++, so nothing here pretends to be a sanitizer verdict. It tests what the interview tests - reading concurrent code and saying precisely what is wrong with it.