This is an example from Anthony's page at http://www.imagemagick.org/Usage/layers/#mosaic. The result is not the expected mosaic, but simply the last image in the command line.
convert -page +5+10 balloon.gif -page +35+30 medical.gif \
-page +62+50 present.gif -page +10+55 shading.gif \
-background dodgerblue -layers mosaic mosaic.gif
The correct result from the example page is
P.S. I get a bad result using -flatten also, where only the last image in the sequence is flattened at the top left corner of the background image.
convert -size 160x160 xc:dodgerblue -page +5+10 balloon.gif -page +35+30 medical.gif \
-page +62+50 present.gif -page +10+55 shading.gif \
-flatten mosaic2.gif
