Using the following as test.fs:
while true do
printfn "unneeded"
true
will produce the following error:
(2,5): warning FS0020: This expression should have type 'unit' but has type 'bool' ....
However, the error is actually on line 3 with the true. This was tested with a build from git of the mono version, but I believe these sorts of bugs go here.