Omitting the @_ can save your day!
Look at the attached file...
> agdachecker bug.agda
bug.agda: Fail: (Position "bug.agda" 33 54,ErrMsg:Error
in the definition of fail1 because:
Constructor type is not a data: ?16)
> agdachecker bug.agda
bug.agda: Fail: (Position "bug.agda" 34 57,ErrMsg:Error
in the definition of fail2 because:
Constructor type is not a data: ?16)
The error message is quite obscure...
Andreas
Documentation of bug