To name a few of the most common:

Unwelded or badly welded groups.

Uncarefull build layered clothing.

Non standard rigging or grouping.

Loose and unconnected parts in the object file.

All will make you work harder then required.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!