Forest fires may also sometimes cause flight crew to lose situational awareness under heavy workload and nearly land at a general aviation airport rather than the planned destination airport.
The incident I'm thinking of is the one at Vernon (rather than Kelowna), British Columbia, Canada several years ago. They had been flying VFR because the full Kelowna ILS approach could not be flown due to emergency restricted areas [due to the nearby fires].
Around that time, there also were serious forest fires in another Canadian province, causing low-vis flying across a fairly large area. No big deal if flying IFR, but VFR wasn't too much fun while flying NORDO. I vaguely remember visibility was approx 1-2 miles at best and was rather hazy. It was also strange to smell burnt air and see bits of ash hundreds of miles from the source.
Volcanic ash is truly nasty stuff; forest fire ash isn't a big deal per se other than reduced visibility when flying VFR. I'm not sure what bush fire byproducts are like.
EDIT: clarified why the Air Canada crew didn't fly IFR into Kelowna.
[Edited 2006-12-11 11:56:02]