Yes, transmission is possible, but not likely.

It is a fact that HIV has been found in pre-cum samples in the lab.

It is a fact that there have been cases of oral transmission where the source did not ejaculate in the mouth, hence it is assumed that the pre-cum was infectious enough to cause transmission.

It is also a fact that unless the guy is a heavy leaker, there will generally be less fluid in pre-cum than in ejaculate and therefore, the risk of transmission from pre-cum is less.

Although, and here is where conjecture comes in, if a guy has a bacterial urethral infection (let's with Gonorrhea or NSU) then he will have a lot of white blood cells at the tip of the penis. If he is infected with HIV, these white blood cells will probably have lots of virus in them. During sex, the first fluid to push out those white blood cells cells (puss really) would be pre-cum, so in this case, pre-cum could conceivable be more infectious than ejaculate.

Don't be hard on people who "slip up." It's not a question of being "smart enough." When people have sex, they almost always rely on non-verbal communication. This is not likely to change, no matter what HIV prevention messages tell us. People just don't ask, "Do you want to fuck?" But they will put their penis up near the vagina/butt and see what happens. At this point it is often hard to stop and ask, "where are your condoms?", or "wait a minute, I need to get something." So slip ups are going to occur, unless you are very careful to avoid letting your partner enter you unprotected.

To minimize the length of the slip up, you should have condoms physically within your reach, i.e. right next to you. If you don't have condoms, you should have outercourse using hands, mouth (without cuming), legs, lower back, chest, whatever of your partner. You might find you like that better because it takes longer to cum, thus extending his/your pleasure.