This happens to me today when I was trying to add a skin using the file association (Win 7 32bits) and Rainlendar2 running. If Rainlendar2 is closed and use the file association or if I drag&drop on a window then it works right.
I have another picture of a more stupid error. I was making a script to download the date of an event from a website. It works fine, but 4 call stack dialogs appears. The error was trying to create a time upside-down: os.time{year=10, month=8, day=2012}
Of course, wxWidgets yell out starting with IsValid() and "JDN out of range".
It worth the mention.