i was playing as Magdalene trying to beat the full game with her, never doing that again btw, and i reached the womb level 2. Earlier I got a Emperor Tarot card (and a crucial 1 up) and i held onto it until then so i didn't have to figure out where in the womb moms heart was. I killed mom's heart with a 1/2 heart left and when mom's heart died, a sucker exploded and killed me! I was teleported back to the first room and had to fight my way to the heart and fight it again! I won the second time ><
Same thing happened to me with Monstro II one time. For some reason, as I picked the Baphomet spoils (with the cat) and died, Monstro respawned as I entered the room again... and, I actually got two boss spoils from him that way, so it was a useful glitch :P
That happened to me once, but with Mom, not her heart. I had the cat (unaware it revives you with only 1 heart) and ended up dying like 8 times to Mom. I finally beat her, than found out there was 2 whole levels left, and cried. (Also, am I the only person who thinks Mom's Heart is so much easier than Mom, and all the other bosses as a matter of fact?)
An interesting note: once after beating Mom's heart I left the room instead of opening the chest, and at that point it gave the message about the item you get for beating the game so many times appearing in the basement, rather than waiting for after the credits.