I recently wrote a tool called SharpFuzz that enables fuzzing .NET programs with AFL (https://github.com/metalnem/sharpfuzz#trophies). It has found over 70 issues so far in various libraries (including the .NET standard library). The most common ones are unexpected exceptions (for example, method that should not throw anything throws IndexOutOfRangeException or NullReferenceException), but there are also many serious ones, such as temporary/permanent hangs, stack overflows, and process crashes.