I lost 200 Odins last night because I was going through my litany of heroes exchanging Helas for new stuff. It's WAY too easy to accidentally hit the regular buy a blessing recipe instead of exchange recipe, especially in the haze of doing this for dozens of heroes.
You really should not let some one buy the regular blessing recipe when it would cover up an existing blessing, or at least give an "are you sure" message. I'm pretty bummed about losing those Odins.