You’re playing along with an improvised break and you just used that right finger or thumb when you realize that you need it for the very next note – a JIT Failure. JIT stands for ‘Just In Time” and is a standard concept in IT and manufacturing. The idea is to not waste storage space for things in inventory; just have them at the very last moment and so minimize the expense of overhead. Our picking fingers are just that way as well; we need them at that exact instance, and not before. But if you’ve just used a particular finger, you can’t use it again if you’re playing fast!
What I like to be able to do when this happens is go ahead and make room for the quarter or half note rest that ensues, and work that extra bit of delay and syncopation into the break somehow. If you can pull that off, it actually sounds even nicer; no small part being due to the spontaneity of it.