In the standard uploader, color the background of the ERROR button yellow when it appears
More than once, I've clicked "Done" to exit the uploader, only to notice just as I do so that the "ERROR" button has appears (I think with the number of errors). Best I can tell, I've lost my chance to determine which upload items didn't succeed. A yellow background for the Error button would increase the chance that I'd click it before I exit. (A "Want to check on those errors before you exit? Y/N" message when I try to exit would be even better.)
David Kinder commented
The standard uploader does let you upload files and skip duplicates -- that's how I've overcome the "missing the error" problem you describe. Simply upload all the images again (or at least the new images you were adding that had an error along the way) and let smugmug skip duplicates automatically.