I was playing a game last night on Magic Online and had attacked with a Hardened Berserker holding up Death Wind and 1 black mana. After blockers, I intended to cast Death Wind for X=1 on my opponent's blocking 2/4, but the GUI did not seem to allow me to cast the spell for anything other than X=0. As cost reductions are applied after costs are determined, my understanding was that I should have been able to cast the spell for X=1 while only playing 1 black mana.

Am I correct or is Magic Online? I reported this as a bug but was informed that it was functioning as intended.

Edit: I just wanted to follow up on this clarification request. I did some further digging and replied to the customer service team and it does look like this was a bug. I've quoted the relevant paragraph from the comprehensive rules below.

107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

In addition, per 601, players should first be able to announce the value of X and then pay for it taking any cost reductions into account.

Edited Rich Marin (April 6, 2015 10:05:58 PM)

Hardened Berserker's cost reduction does indeed allow you to cast Death Wind for X=1 with a cost of just {B}.

The problem here, however, might be a confusing interface rather than an outright bug. After checking out the interaction, MTGO does indeed allow you to apply the Berserker's cost reduction to Death Wind, but it does so in a weird way that doesn't match up with how the interaction's supposed to work, by asking you if you want to apply the reduction to X (which it shouldn't–the reduction's not optional) before it gets you to determine the rest of X by paying for however much you want it to be. You may have accidentally skipped over the reduction part of the process, and then been stuck with X=0 when you couldn't pay for more.