First off, keep playing. It's not great when the music is off from the dancing, but don't let your efforts to fix it distract you enough that you stop playing danceable music. Watch the dancers, see when they're starting things, and try to figure out how far off you are. When the dancers start new moves, does it fit the beginning of a phrase? If you don't remember what figure the dance started with try asking the caller where that is.

A few beats: Differences of a beat or two tend to resolve themselves with the next swing. People swing until they hear the phrase ending. So if people are staying off from the music through multiple swings there's probably something wrong with the tune you've chosen or the way you're playing it. First try really emphasizing the phrasing, maybe with breaks and really hitting the first beat of each phrase, but if that doesn't work switch to a different tune.

A multiple of 16 beats: You're probably playing a tune that's AABB, with both the A and B parts being 16 beats, so this is the case where the dance is on the A1 when you're on the B2. This is the most common mistake, and usually comes from skipping a repeat. Do a little math in your head to see whether it's closer to add another part or remove a part, and then add or remove B parts as needed. Simply shouting A1 right before the A1 is often enough.

An awkward number of beats: Sometimes you're so off you can't even feel where the A1 is supposed to go. The worst is when you're off by four beats; that adjustment is very hard to think about. In this case just start droning, dropping all melody and phrasing and just leaving the beat. Watch the dancers. Once the old melody and timing is out of your head notice when the dances finish a move and start adding phrasing that reinforces what they're already doing. When you get to the beginning of the dance, bring the melody back in with the A1.

When you notice you're off, think back and try to remember whether you had a "something isn't right" moment a little earlier. As a musician you're probably used to simply listening and adjusting when you notice your sense of the music off from the rest of the band's, but when you're playing for dancing you want to learn to notice. Yes, adjust to what everyone else is playing, but also look at the dancers and try to figure out if the adjustment was you coming back into place, or everyone else shifting to somewhere not quite right.