I've found that, when wearing a hat, the brim can
cause an audio reflection. The monitor sound
bounces off the brim on one's hat and into the
front of the microphone.
It's kind of difficult to explain, but it has happened
to me. It's akin to the technique of partially cupping
one's hand over the microphone to see if you're set
on the edge of feedback.
Maybe someone needs to invent a hat with built-in
GOBO.