Birdforum thread on mass hallucinations (or mis-leadings)

Here.

Note this story:

On a field trip a few winters ago the leader pointed out a Snowy Owl on the Lake Erie shore. It was well known among local birders that the lump was in fact a discarded boat bumper washed up on the breakwall. When this was pointed out to the leader he went on about his experience working with Snowy Owls at a rehab facility in Canada and he was certain this was a Snowy. About 2/3 of the people on the trip dutifully checked off Snowy Owl. It was a lifer for 4 people.

It was one extremely patient Snowy Owl as it refused to move from its perch for more than 3 years until the US Coast Guard removed it when cleaning trash off the breakwall one summer.