Worst I have ever had was around 500 daddy-long-legs spiders between the mesh and the rain fly. Harmless but tickled like heck when you try to dislodge them. (They really REALLY wanted back on the tent!)

I have had a few things wander into the tent but nothing damaging (including a bear cub...though he did destroy a roll of paper towel but left everything else alone).

Oh, and for some odd reason, slugs like to squip their way up the guylines and you don't think much about it until you grab the line and.. 'yuck...'