My most frustating error with swiftc is the switch on enum "not being able to complete in acceptable time" (or something like that). Basically, when you want an exhaustive check on combination of enums, you're often reaching the limits of the compiler.

something like this

enum State { case A, … } switch (fromState, toState) { case (.A, .B): … }

in which you want to make sure you didn't forget a combination.