Several times. Couple times I was embarrassed to find out I couldn't help; once on a brevet when my pump's check valve was frozen (and he got his pump to work about the time my armpit heater thawed mine), and once when I couldn't get the reversable plug out of my pump to help her with her Schrader.
Non-flat help best memory was coming home from work, with dyno lights front and rear and two battery-powered blinkies. A lady was shepherding her two children on bikes, so I caught up and told her to keep going, I had enough lights to spare. I tailed them for couple blocks until she pulled into her driveway and explained they were slower than expected, and so they weren't prepared for dusk to come so soon!