I'm not a real SP only a wannabe.
I've had condoms slip off inside an SP during sex many times. Sometimes partially, usually completely off.
Neither i or the SPs ever realized it until i pulled out and, voila, no plastic baggie.
Sometimes they had quite a bit of trouble extracting it.
Usually they were Vancouver low track drug addicted SW SPs. Sometimes Thai SWs in BKK. Never experienced any STIs of any kind from these little "slip ups".
With intentional, rather than accidental, BBFS i've had roughly 100-200 sessions with 20 SW SPs. During this time i had one or two minor STI, although they may have come from BBBJs or some other activity rather than BBFS. They were easy to deal with, in a few days, & i am now cured.
I'd estimate my average session of BBFS lasts 30-60 minutes.