Fired up the Arrow with the bird bungs still in. Quite an impressive shower of foam, and the red ribbon flapping gently over the top of the engine air intake. Picking the foam chunks out was awkward though.
The worst thing was I had also done the walkaround before getting in, but having seen the bungs in all day while waiting for passenger to return I was used to seeing them there, so it didn't trigger anything in my mind.
Lesson learnt - look and SEE, listen and HEAR.
Then there was the one I heard about (thankfully before trying it myself) where someone was about to start a C152 with the towbar still attached and sitting nicely in the propellor arc.