It sounds like a method is trying to implement an interface method, but has the wrong signature. Something like:
Sub Frob(foo as integer)
Sub MyFrob() Implements IWhatever.Frob
Edit: ... Yup, those those are the exact error messages when you compile the above snippet.
It's worth noting that this error cannot occurr in C#, since it determines interface implemenations based on the name, not on an additional decoration. If you screw up the signature, it'll assume it's a totally separate method.