Right.

I don't think this belongs in the Penile problems xyz thread, but if it is, obviously move it in.

So last night the girlfriend and myself were having sex with Lifestyles lubricated ultra-sensitive condoms, she had already came and I was on the final stretch, and when we went to change positions, while grabbing my shaft to guide it in, she stops.
Looks at me, and goes
"Uh, BloodiedMind, Where the fuck is it!?"
She stands up, somewhat in shock, and we both stop, dazed for a moment.
And slowly look towards her vaginal area.

Much probing and some "Oh shit"ing occurs, and we finally get out the renegade condom.

Now, I'm definately in no position to have a child, so we googled it up a bunch and found that the Plan-B pills are over-the-counter and not exorbitantly expensive.

We proceed to the pharmacy today, buy two pills for all of 20 dollars or so (peace of mind for a small fee), just to eliminate the even-small-possibilities that could have occured from pre-come being inside and unprotected.

She's definately getting on the pill, as was the plan prior to this regardless, but just for future reference and that extra-bit-of-protection not going missing:

What am I doing that would cause the condom to fall off? It rolled all the way down the shaft (In fact, I changed condoms before any sexual activity because the first didn't go all the way down). Naturally, I check to make sure it's still on and good whenever positions are changed or anything, but on such times as this where there's a continuous usage of a single position (can't change so close to an orgasm!), how can I ensure it stays put?

Short of buying myself some Duct Tape, that is...

Smaller than you thought you were?

try different ones?

theres a lot, its hit and miss, only thing u can do is try em out, and see what works best.

Well, in my case, I usually don't stay 100% erect throughout sex. It may go a bit limp, then erect again, etc.. which causes it to get a bit loose. Your best bet would be trying other condoms(Ultra thins don't play nice with mine) and making sure that it's ALL the way down. And you know, get a little feel every now and then to see if it's still on.

It happens man. :P

That's the thing, though. The condom doesn't feel loose at all. Not even a slight bit.
It's definately tight when I roll it down the shaft. Occasionally a little uncomfortably tight, but's momentary and passes quickly.

So unless I'm crazy, it doesn't seem that it's too large at all.

Maybe it was just a one time thing?

I'm not sure, maybe when you were changing it rubbed against something and made it come off?

No idea.

Maybe your girlfriends vagina has teeth?

http://www.cswu.cz/basic/images/sarlacc.jpg