Spurious image during screen share

[PeterR]

During a meeting yesterday one participant (not the host) was screen sharing a Powerpoint presentation.

All participants except two saw the correct images.  However two participants have reported seeing a different static image during the presentation (it seems from their descriptions that it was the same image).  In one case the image persisted through the presentation; in the other case it came and went.   Nothing like this ever before.  Can anyone offer anything by way of explanation?  (that doesn't include a reference to Halloween!)

Peter