I accompanied a buddy flying his dad's 150 many years ago. it was the first flight in a long time (hence my mentoring). As we cruised along, it began to get warm in the cockpit, so he pulled open his vent in the wing root. It had obviously been closed for some time, as once opened, the cockpit filled with bees. To make it worse, he instantly announced he was allergic to them.
With both windows opened, and a door forced as far open as I could, we got some of the bees out, and landed back in a hurry, carrying the rest who seemed happier in the back. it must have been a sight, with us rolling off the edge of the runway, and getting out before we'd even rolled to a stop!
Now, for the first flight of the season (or a long time), I open those vents on the ground first, if I think I might need them in flight!