http://gumroad.com/l/50-modeling-issues-from-hell

3D modeling is hard, sometimes needlessly uncooperative. So many things can go OFF THE RAILS. Even if you feel more or less comfortable with 3D modeling in Blender, there are always more things that can spoil the fun.

Common 3D Modeling Problems in Blender
A subdivided object collapsing into a shapeless mess
Topology going haywire
Non-manifold geometry
Boolean ops ruin the topology
Some models just explode (due to an error in the modifier stack)
Bevels overshooting
Exported models are broken
Too many polygons to even move a vertex
...or Blender just crashes