Player A has Mercurial Pretender on the board (copied a 4/4 creature)
Player B attacks with Typhoid Rats
Player A blocks Typhoid Rats with Mercurial Pretender
Player A then pays 4 mana to "Return this creature to his owner's hand"
Player A returns Mercurial Pretender to his hand, then says Typhoid Rats dies and goes to graveyard.
Player B thinks that player A cannot both successfully block and return Mercurial Pretender to his hand, considering Typhoid Rats Deathtouch ability.
Who is right?
As noted by Hackworth, Player B is correct under the current rules that the Pretender can't kill the Rats if it is returned to hand. However, it did still block them successfully and the rats won't deal combat damage to Player A or anything else this combat.
One possible source of confusion in the situation is that it used to work the way Player A thinks. You could "put damage on the stack" then play abilities in the combat damage step in response, before the damage resolved.
This was removed with the 2010 major rules update (the one that also changed Lifelink and Deathtouch). Now, player B is correct, because there is no place to respond between assigning and resolving damage.
Player B is right.
In each combat damage step (First strike/double strike, then other creatures), all creatures deal their combat damage at the same time, and you don't get the chance to cast spells or activate abilities:
510.2. Second, all combat damage that’s been assigned is dealt simultaneously. This turn-based action doesn’t use the stack. No player has the chance to cast spells or activate abilities between the time combat damage is assigned and the time it’s dealt.
Neither is correct.
Player A is wrong because he returned his blocker before combat damage was assigned and dealt therefore the attacker would not die.
Player B is wrong because player A can successfully block AND return his creature. The attacker's deathtouch ability is irrelevant.
