Always, even racing I'll throw out a wheezing "you okay, need anything?"

I had a guy lend me a chain tool during a race once (me thinking it's a short XC race, I don't need my blowout bag ) I've also taken a tube from a guy at a local trailhead.

It's always a good idea to stock up on bike karma. I've been driving, and having witnessed a presumably new roadie (new b/c he was wearing a sweatsuit) walking his bike on the shoulder of the road. I stopped, gave him a tube, changed his tire and gave him some pointers on how to do it for himself. I've given out tubes to other roadies, but I actually don't recall another MTB'er needing one yet.