How forgiving are agents of the occasional typo?

Small, rare typos are just fine--I had a couple in mine, but my agent pointed them out when we did revisions. As long as it's not massive, you should be ok.

If it's an occasional one, don't even stress out. We all have them. But if it's pretty obvious you didn't spell-check or even bother to read through it? Well, that's another story.

The main thing is that your MS seems professional and well-polished. Perfection not required . . . just close.

I had several (including an incorrect use of your - you're) that slipped by in the full manuscript. When the agent called to chat I of course groveled about the typos and she just laughed and said it happens. She offered representation anyway.
If your manuscript is littered with them -- watch out-- an occasional typo is unavoidable.
There are much better things to obsess about -- like the premise and the writing.

Typos

Any typo at all is bad in a query letter, but the occasional typo in a manuscript bothers no one.

Maprilynne -- aren't you supposed to be having that baby instead of messing around on AW?????

I have a good story out here, though don't ask for names, pl. Here I was, with an Indian publisher but no agent to negotiate for me. So I query and I query and a top agent bites. I ask her to send a mail to the Indian commissioning editor so that they can start talking. She sends in the mail with a copy to me. To my horror, I find the first sentence lacks a crucial word. I give her hell. She says sorry, yes, but adds it's very American to be casual. I sack her. For all those with an affinity for having been thrown into the slushpile, this one should be a confidence-booster. Since then, I have had to apologise to my Indian publisher for that mail she sent out in a hurry and have got myself another agent in a day's time. Life's not all that sad out in the agent world, believe me. Cheers.