Make sure that you are using "-w", and hopefull strict. This increases the likeliehood that you will be able to provide a useful error message
Corrolary to Show the Code
Show the Data
If you have a sample data set, provide it; If the set is too large, provide a sample piece of it. If most of the set works but a particular subset doesn't, provide a sample of what works and a sample of what does not.