Someone mentioned this to me on Twitter (hooray for functional Twitter!) when I asked whether there was a name for a particular turn of conversation around a contentious topic. I'm afraid I forget the topic, but the conversation's form resembled the following: person A said "here's a problem," then person B said "that's not nearly as bad as this other problem."
What's wrong with that? Nothing, inherently. What would be wrong, though, would be the reader, if she thought "problem B is the worse problem, so problem A is therefore not a problem."
Indeed, I had the impression this might have been person B's implication. Strong emphasis on might, of course – I could have tried to find out by asking directly, and one of the commonest and worst mistakes available when discussing on the Internet is to jump to conclusions about what your conversational partners mean. (Instead, I raised that question about the fallacy's name.) That's also why this fallacy, like other fallacies, might be slipperier to identify: for someone aiming to use it, it could be slipped in to dialogue under cover of ambiguity.
(Note: this also seems to be what I've sometimes heard Americans call "whataboutism.")