Not necessarily a red flag, but be sure to ask why they are last minute when they come for an interview (or in a phone interview first). All kinds of things can happen...people can suddenly get a job offer and decide to go back to work earlier than planned, they could have had a daycare lined up that fell through (this happened to one of my clients...daycare provider cancelled on her 2 weeks before she was to return to work), etc. I would certainly not write them off.