Recently I responded to a newsgroup post where the Sales Transaction Entry (SOP_Entry) window was popping up the "Do you want to save?" dialog when moving between records even though no changes had been made by the user.
I have seen this issue many times caused by Visual Basic for Applications (VBA) code added to the window. Usually, the issue is caused by the code on the Window open events or the primary key field changed event. This code is setting a default value or some other field on the window, and as a side effect setting the window's changed flag to true.
The solution is to add the line Me.Changed = False after any code that changes field values to clear the window's changed flag.
24-Jul-2012: Add Link to related post: Quick Tip: Solving "Do you want to Save?" dialog appearing after VBA customization.