After uploading an image to this Mediawiki installation, the thumbnail wouldn't be displayed:

   Error creating thumbnail: Unable to run external programs in safe mode.

Hm, it probably tried to run imagemagick to generate that thumbnail, but safe_mode was off anyway, so what gives? As it turned out, the passthru() function was disabled. After removing it from disable_functions thumbnail generation was working again. However, I'm not sure about the security implications yet...