Lugging an engine

This thread http://boards.straightdope.com/sdmb/showthread.php?t=279472 made me wonder about another issue, lugging an engine at low RPM. I’ve been told all my life not to do this, particularly with small displacment engines. I know that in some conditions that loading an engine at low RPM might result in detonation but aside from that what is the mechanism for engine damage or excess wear if any?

Seems to be primarily a damage via “prolonged & heavy detonation” issue