I was flying a helicopter on an IF training sortie once where the lower half of one side of the windscreen was covered with a screen, so we had a crewman in the back covering the lookout on that side.
We were in and out of puffy Cu doing an NDB approach as I recall, when the crewie came up on the ICS and said in a kind of funny voice something like "I'm sorry, I don't want to be up here any more. I need to land."
Not those exact words probably, but that was the gist. So we landed and he went to the boss and withdrew himself from flying duties. He couldn't really explain it himself, and had previously had no troubles doing all the whacky things crewmen are required to do, hanging out on the hoist and all that stuff.
I don't know if it was that particular sortie that set him off, or a cumulative thing that caught up with him.