Time was when that would mean that I would promptly fall over, but that is now an exceptionally rare event, fortunately. Nowadays, I'm much more likely to be able to catch myself or to be positioned near something that can take my weight. It is still a rather negative event, and one that I would like to avoid, if possible, and understand. Naturally, I think I have an idea what is going on.
Periodically, I think my brain tries to route the command to my quadricep the way it did for 40 years. That chunk of brain is long since dead, of course, so the instruction fails and my leg buckles. This would seem like a pretty daft thing to do, but I don't think it is. One of the scenarios that causes things to fail in the brain is swelling. (I'm sure there are lots of others, too.) If all goes well, swelling goes down, and normal function can resume, as soon as the brain figures out that it can use the hitherto blocked pathway again. If it doesn't go well, you die.
So an algorithm that periodically checks an old, non-working pathway makes sense, because there may be no other way for the brain to establish that a path is working again. This would be much more useful to me if I didn't periodically buckle, but such is life!