Of the different types of bad time in jams, here are the ones I've experienced the most:
1) The (usually banjo) player who starts the tune with an instrumental introduction, but he can't really quite play it, so he plays it one.note.at.a.time and there's no hint of what the actual tempo will be. It usually takes the first stanza of full-ensemble playing for a tempo to be established. This is distinct from the "aimless strumming with no information" I describe below, because it does actually turn into a tune with rhythm, harmony and melody; it's just the guy doesn't know how to get it rolling with his intro. If I know the player well, I'll yell "Fewer Notes! More Quickly!"
2) The soloist who practiced the note-for-note solo at home so long that he can only play it that way - either too fast, or too slow - or sometimes, both. In that case I try to make LOUD short notes exactly in time to keep the rest of the group on track. If the soloist takes too long to finish, he'll still be squeezing out those last few notes while the singer rips into the next verse, or if he rushes through it too fast, there'll just be a little pause. Worst thing to happen is when the group splits, half trying to follow the soloist and half trying to keep on track. If the soloist gets too far away from the changes I may just play root-root-root-root just so everyone can HEAR where the changes really are.
3) The guy who says he's going to lead the tune, but he doesn't know it, can't play in time, doesn't know when the chords occur, there's just some aimless strumming. I'll try to drop bass notes in to see if the band can pick this thing up and make it a going concern, but after a couple tries I just lean on the bass and wait for it to be over. It's usually over pretty soon, mercifully.
4) The whole group gets revved up and speeds up through the length of an exciting tune. This is one where you're not going to hold 'em back, you've just got to hang on.
Someone up above pointed out that the average amateur bassist is probably spending orders of magnitude more practice time with a metronome than any other bluegrass instrumentalist, and I'd say from what I've seen that it's probably true. I have trouble understanding how, after 70 years of the predominant popular music all over the world being rock and roll with its powerful explicitly defined beat, people claiming to be musicians still can't hear where the beat is, but it's so.