Fail to stop at stop sign

[rubixcube]

but you may have been just short of the line (say by a metre) and then they've observed you roll over it.

If you stopped a little bit short of the line, you still legally stopped. It's "as near as practicable". To me 1m is pretty close. As long as your wheels come to a complete stop and you aren't a whole car length away from the line you should be golden.