The current implementation seems to be "if your outstanding friend requests are maxed at 50, any new requests silently disappear into thin air".
Which can be very annoying if you PM someone here on the forum asking to friend you, and then the list get capped by request spam before that person can send you the friend request. (or the other way around: PM: "I sent you a friend request", reply: "never got it, since my outstanding list was capped")

If should be made first in first out, where a new request pushes out the oldest outstanding request when you're at the cap.