The Swift compiler segfaults very frequently. I do find this amusing in that it's the compiler for a theoretically largely-memory-safe language (yes the compiler is written in C++, it's still funny). The syntax highlighter in Xcode, which is driven by the same stuff, also crashes, which breaks autocompletion and even indentation. Using Xcode, you just have to get used to it. It frequently reports the wrong error message - just something that isn't even close to related. Sometimes object files just become 0 bytes and you either need to clean (and experience the Swift compiler's blazing performance again) or go and modify that file so that the incremental compiler will pick it up.

I've found most of these to be triggered by using a lot of closures and possibly type inference. Shaking out the incorrect errors or segfaults is... not fun.