These types of offers are definitely not old wives' tales or urban legends. In fact I think they are quite common among the residency programs in anesthesiology.
I would describe the types of spots offered as being one of two types:
1) Program is "average" or does not always fill, and keeps spots reserved for a strong DO or IMG candidate.
2) Program is "elite" (for example, a Harvard program) and keeps spots reserved for a strong FMG (not IMG) or for someone who boarded in another specialty or is transferring from another specialty (for example, plastic surgery resident who pulled excellent numbers in medical school and no longer wants to continue plastic surgery).
If your offer is from an "elite" program I've known people who got offer at X program (very well known) but would prefer Y program (equally well known) but Y program refuses to offer them an outside-the-match spot and tells them to go through the match instead. I have known these types of candidates to actually turn down the outside-the-match offer and then match to their true top choice program through the match. They put you in a tough spot but usually if you're that type of candidate, from what I hear you end up matching well anyway.
If your offer is from an "average" program, also puts you in a tough spot.... they've made you the offer and put time pressure on you to sign with them and give up your other prospects, when you don't know what your prospects really are.
BUT.... I have also known a candidate who signed a pre-match agreement, went through the match, found out that HE MATCHED at a place he wanted to go to, and told the pre-matched institution he was no longer coming. I do not know the full details so can't answer many questions about how he did this -- all I know is that he did it.
