What are the possibilities of catching anything after having anal sex with a person (with no condom) — even if s/he says s/he doesn't have AIDS — for only about ten seconds and stopping with no ejaculation occurring?
Word of mouth is not a good way to determine whether or not your partner has a sexually transmitted disease (STD) or infection. Remember that one of the most common symptoms of a STI/STD is no symptom. This is also true for HIV/AIDS. A person can be HIV-positive, but show absolutely no signs for years. A person could also be HIV-positive and show up as HIV-negative on a blood test if it is during the "window" or latent period before the virus can be found in the blood. (Read HIV Transmission: When does it show up on a blood test? in the Go Ask Alice! Sexual and Reproductive Health archives for more information on the window period for HIV.) Your partner may not honestly know that s/he has the virus (or any STI for that matter) at the moment when you're going to have sex.
It is possible, but unlikely, that HIV (or another STI) could have been transferred from the inserter to the receptor during the very brief period of anal sex you described, through the inserter's pre-cum. It is even less likely that the receptor could have transmitted the virus to the inserter in that short amount of time, unless there was tearing and blood in her/his anus. Any type of sex can be risky, but using barrier methods (such as condoms) cuts down this risk significantly.
For the future, if you want to enjoy anal sex with a partner safely, use a condom from the very beginning of your sexual encounter and use lots of water-based lube.Alice!