A couple of years back I had one go "Cheers then, I'm off" a full hour before the first dance.

Me "You what?"

"I've got a train to catch"

"But we haven't done the first dance yet"

"Not my problem"

Now granted we were running about half an hour late, but this was still half an hour before the scheduled time & 20 minutes before cake cutting was due.

The bride & groom's first dance photos consist of stuff the videographer & I took between doing what we were supposed to be doing.