Yeah, happens to me too.
If I scroll like hell up and down it keeps scrolling for a long time after I stop. The same does NOT happen on other SMF forums. But same thing happens in other Gecko based browsers, such as Epiphany, but still only on ZFGC, AFAIK.
EDIT: And no, I have smooth scrolling off (always had). It's pegged on 100 % still. When I enable it though, the effect of stuff overlapping as Sol described is much more visible.