[jacobreg]

Altitudes flown aren't independent of heading. Depending on whether you are VFR, IFR, flying magnetic east or west, you will fly at specified 500 foot increments.

https://www.law.cornell.edu/cfr/text/14/91.159

Edit: I read up a little more about the accident, and it happened above where these rules apply. In any case, with problems like this where one plane isn't where it should be, I think the solution is to come up with better ways of ensuring clearances are complied with, not coming up with a system that is more tolerant of people violating clearances. Airspace is only going to get more congested and the only way do accommodate that safely is to have planes follow clearances to stricter and stricter tolerances.