Some examples of what I'm talking about:
... You [not trans] have heard that "tr*nny" is an offensive term, but a transsexual woman you just met says she prefers it and is offended by your attempt to question her identification with it or use another term.
... You [not a woman] think that a comedian made a misogynistic rape joke, but your girlfriend says it was funny and made her laugh.
... You [not a POC] mention your objection to racist Hallowe'en costumes to a POC friend of yours, and they say that you're making a mountain out of a molehill.
... In the general case, a member of any marginalized group that you are not a member of disagrees with you that this marginalization or a specific manifestation of it exists and/or is problematic.
In all of these cases, I would suggest that you take this person at their word and not challenge them or push them on it the way you might if it were someone who was not actually the target of any of these things commenting on it, like your cis friend who's in love with the phrase "hot tr*nny mess" or your white friend who can't wait to dress up like an "Arab Terrorist" for Hallowe'en.
Because it's pretty ironic to tell someone on the receiving end of systemic social injustice how they should feel about their own life in the name of social justice. Because it's not helpful to force an impossible standard of universal consensus on a group as a prerequisite to taking them seriously. Because there are only so many hours in a day and only so many shits a single person can give about their own life - let them choose which shits to give and where. Being angry and offended takes energy, and people aren't going to spend it all in the same place, especially if they are already fighting a lot of battles in other ways that they are being crapped on by the kyriarchy. And because people just plain disagree - it happens.
What this also means (and doesn't mean):
1. You don't have to automatically agree with this person's opinion. In the last case I raised (which is roughly similar to one I experienced this past week), I didn't change my mind about the harms of racist Hallowe'en costumes, largely because I do find the claims made by POC with different views persuasive - like Thea Lim's take or Adrienne's horrifying gallery. In the first case, which I also based loosely on a similar experience, I didn't raise an objection, but declined to use the term myself. (Note: It's entirely possible to disagree with someone without being dismissive or condescending about it - strive for this.)
2. Even if you do agree with them, they don't become your magic shield against being challenged on your own crappy behaviour. This is the token friend derailing tactic, which Renee recently took apart at Womanist Musings. Basically, opinions don't cancel each other out, and if you are doing this, you are also not an ally in any sense of the word.
3. The fact that people within the same marginalized social group disagree is not grounds for dismissing any of their opinions or the whole discussion altogether. See what I said above about inappropriate and impossible standards. See also: Renee's post linked above. It's quite simply unfair - when was the last time you needed to demonstrate consensus of your gender, racial, class, etc. group in order to be taken seriously?
One of the major traps that people who want to be allies fall into, especially when we are very privileged to begin with, is a desire for things to just be simple. We want to learn the rules, play by them, and win the social justice game! Yay!!! No. God no.
(See: mai'a's we don't need another anti-racism 101 for some of the implications of this tendency.)
It's a dead-end way of thinking because things just aren't that simple and won't ever be. These are big messy issues being hashed out across multiple spaces and contexts in evolving conversations whose boundaries and terms need to be defined by the people with the most at stake in them, not the people who are calling themselves "allies". Even for this post, for all that I've structured it like a guide, I'd be lying if I didn't say "exceptions probably apply", some of which I'm sure are beyond my imagination at the moment. There is no easy formula for being a useful ally.
There's also an underlying issue here of insisting to someone that they are marginalized, victimized, disempowered, etc. in a way that is actually marginalizing, victimizing, and disempowering itself, while we, as the wannabe allies, cast ourselves as some kind of enlightened saviour. In which case, rather than breaking down a power dynamic, we have reinforced it.
So sooner or later you will find yourself in this position, if you haven't already, and, caveats firmly in place, my advice to you is the same fundamental advice that's always given: shut up and listen. Respect the person who's speaking and don't assume that your own experience triumphs, regardless of what that experience is. De-centre yourself and don't make it about your need to be right or do exactly the right thing. Maybe, depending on the circumstances, you can pursue the conversation further, but consider the idea that sometimes you just need to accept the ambiguity of the situation and let it go.
Easier said than done. Ah well.
EDIT 02/17/12: Case in point.