The Single Equal Sign Of Doom

That feeling when you realize your production server is granting admin access to literally everyone because you used = (assignment) instead of == (comparison) in your if statement.

Fun fact: This single character mistake is why some senior devs wake up in cold sweats at 2AM. The code if (user = admin) doesn't check if user equals admin - it assigns admin to user, then evaluates to true because admin is truthy. Congrats, you just made everyone a superuser!