These are the options I consider:

1. Shorten the repeat distance. If I came in intending to do 100s at mile race pace, but find myself failing early, I'll go to 75's at the same pace after the first fail. May even go to 50's after a second fail. Or I may pre-adjust the distance before starting the set if warmup feels crappy.
2. If my legs just aren't there from the get-go from recent hard running/biking; I'll throw in a pull buoy and go long-and-strong.
3. I may do drills and/or work on off strokes. 25's with lots of rest.
4. If it all feels like junk, I'll pack it in early and go home.

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"They're made of latex, not nitroglycerin"