One of the biggest causes of errors occuring exclusively in the run-time version of an application is the environment. Alot of settings that we take for granted in our development environment aren't the same during run-time.
For example, in Tools|Options you might have deleted records being ignored, but if you don't explicitly set this in code with "SET DELETED ON", you could be allowing invalid or test data to resurface.
I hope this helps.
Tyson Bonn