I have a model of a small dinghy with a nice mesh occupant that for some reason show as expected in the rendered preview mode but when I start the Cycles raytraced mode and let that display start to run and then switch back to CTRL-Alt-R for the render preview the dude gets mirrored about the groundplane.
Looks like this
Looks like this in raytrace display mode.
You have a reflective ground plane. Your person is most likely a block instance.
This was reported some time ago: RH-62541 Block instances become invisible in Rendered mode with reflective material on ground plane, through investigation for Material assignment with blocks - #14 by hwansey
Anyway, I think it is related.
I think you are right about that Nathan as my guy is a block but with the ground plane turned on I thought he had disappeared as in that report. I bet though instead of invisible he is hidden under the ground plane. Funny thing is when I did a Copy to Clipboard of the image and pasted it here the inversion got corrected. I had to try that again and then I noticed that in the dialog box preview that pops up it is correct and I had to do a screen grab to show the issue. Look again and you can see.
Thanks for your reply and I might try a non-reflective GP to see if that makes any difference.
A white matte groundplane still does the same thing with mirroring the block mesh.
I exploded the mesh block and the mirror thing goes away as you suspected. The guy really looks happy now and so am I!
If you change the ground plane to a non-reflective material while the block instance is still invisible you’ll have to click in the empty space where the block instance is supposed to be to select it. After this your block instance should stay visible even as block instance.
I do that in this screencast that I linked in the YT item as well