My DH and I have different last names. The last two reservations I made they listed my DH with my last name. I did make a second call to MS to fix it because I didn't want to tie up the line at check-in trying to get it fixed there. Since we pay for everything with our KTTW cards I thought it might cause problems if his signature didn't match the name on the card. On the other hand, it might have been interesting to see if anyone noticed!
I'd lump "getting the last name wrong" under egregious spelling and probably call to fix that one.
Now, we have had the HORRIBLE spelling issues with ADRs, where finding our reservation has been "match SOME of the letters with the time we have our ADR for." Linking ADRs to DVC/CRO has helped a lot.