>That sounds like a gotcha in the VB implicit conversion rules with Option Strict OFF. If you have two overloads (one accepting a string and the other an integer) and attempt to call that function with a decimal value as the parameter then the compiler doesn't know whether it should implicitly convert to a string or an integer (ie which overload to use). You should cast to the type you wish to use prior to the call.
>(In C# the same construct would give the usual 'invalid argument' error).
>But I'm not sure why you saw this only at runtime - the compiler should catch it?

That is exactly that. Since I added an overload for a double last night, it started to work.