If you count ASP.NET, I'd say the webforms lifecycle is a pretty big gotcha to me. I've spent countless hours debugging poorly written webforms code, just because a lot of developers just don't really understand when to use which event handler (me included, sadly).
For C/C++ programmers, the transition to C# is a natural one. However, the biggest gotcha I've run into personally (and have seen with others making the same transition) is not fully understanding the difference between classes and structs in C#.
and you find out content.Length is still 0 after all the writes!! You then begin to rigorously checks that you don't have a race condition when.... after one wasted hour... you figured out it's just that tiny little Enumerable gotcha thing you forgot....
Capture the purrfect photo of your ferocious feline friend on their birthday or gotcha day. Tip: Use treats to entice your cat to cooperate during your shoot. Share your best shot on social, and/or send it off to be printed and framed.