Update: look at the expression for Default Value of [Part Number] on the subform. If you copied the form from a posted solution db, it might be referring to "Copy of sfrm_parts" when it probably should be sfrm_parts. My suggestion to those who modify OP posted db's would be to copy any objects you wish to play with, and play with the originals rather than the copies. Control references, code, expressions - all sorts of references to the original object name can be broken unless you modify them to point to your copy. Don't see the point in that 'cause it's just unnecessary work if you play with the original. Here's one example of where an expression is no longer valid because the form name was changed and the original is gone.
Code:
=[Forms]![frm_component]![txtComponentPrefix] & (DMax("[NumericPartNumber]","[qryNumericPartNumbers]")+1)
Gotta take a break for now, so I can't say if the expression will work if I fix the reference. I found what I think is another error in the code for the user_id combo event.