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Describe the bug:
If you activate Grenzo, Dungeon Warden's ability and a replacement effect like Leyline of the Void's or Darksteel Colossus's prevents the card from getting into your graveyard, it keeps putting cards from the bottom of your graveyard until one does stay there. For Leyline of the Void, this exiles your entire library, like it's supposed to with Helm of Obedience (and nothing else, AFAIK); for Darksteel Colossus, if it's the only card in your graveyard, it causes an infinite loop.

Which card behaved improperly?
Grenzo, Dungeon Warden.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Dev 6d9017f.

What exactly should be the correct behavior/interaction?
Grenzo, Dungeon Warden doesn't have a "until [X or 1 or whatever] cards are put into your graveyard" clause like Helm of Obedience, so it always moves exactly one card there. If a replacement effect keeps it from getting into the graveyard, then the second part of the ability won't be able to put it onto the battlefield since it's no longer in the expected zone. (Compare Cellar Door, where if it puts a Darksteel Colossus there, you still should get a Zombie even though the Colossus never makes it into the graveyard. I think. Any passing judges, feel free to chime in.)

Are any other cards possibly affected by this bug?
Sort of. Helm of Obedience probably has the same problem in release with a library containing just one or more copies of Darksteel Colossus (and/or Blightsteel Colossus, Legacy Weapon, Progenitus); if it were able to resolve in dev, it definitely would. What I think is supposed to happen here, based on the third ruling for Wheel of Sun and Moon, is that the Helm tries to put all of the cards into the graveyard at once, the Helm stops since the library is empty, and then the cards (which are in non-library, non-graveyard limbo) have their replacement effects kick in, reveal themselves, and shuffle themselves back into the library. Or you choose which replacement effect to apply, if you've got some pathological combination of Wheel of Sun and Moon and Leyline of the Void and whatnot that are also trying replace cards going to the graveyard. Similarly, if you have one Darksteel Colossus in your library, it's on top, and you cast a Glimpse the Unthinkable targeting yourself, you shouldn't put the same Colossus into your graveyard more than once, no matter how badly you shuffle your deck or how many other cards are in it. Things are much worse in Manalink, since normal-speed mill triggers like on Emrakul, the Aeons Torn or Narcomoeba happen immediately, in the middle of Glimpse's resolution.

Korath wrote:Compare Cellar Door, where if it puts a Darksteel Colossus there, you still should get a Zombie even though the Colossus never makes it into the graveyard. I think.

Since Darksteel Colossus never hits the graveyard why should Cellar Door which needs the creature card to hit the graveyard make a Zombie? This seems pretty obvious.
Anyway if I understand correctly Leyline of the Void’s 2nd ruling clears this out:

15/8/2010 Leyline of the Void’s second ability prevents cards from ever reaching your opponents’ graveyards. Abilities that would trigger when those cards are put into an opponent’s graveyard (such as Hoarding Dragon’s last ability) won’t trigger

It's not an "ability that would trigger when those cards are put into an opponent's graveyard" - it's neither a trigger nor dependent on the card actually entering the graveyard, just on whether the card was a creature card (which the replacement effect doesn't change).

Chiming in - Like you said, Korath, you get your Cellar Door zombie. The way it's worded, the card only cares if the card on the bottom of your library was a creature card, not where it goes.
Interestingly enough, Grenzo only tracks the creature to the next public zone it goes to. So if a sufficiently large Grenzo flips a Darksteel Colossus, you don't get it since it goes into a private zone (the library) - same with any other creature while you're enchanted by Wheel of Sun & Moon. In the case of Leyline of the Void, you do get the creature since it goes into a public zone (exile).