(1) Do not stack caltrops on top of each other vs. PCs

(2) Contact a DM if you're using caltrops within visual range of non-monster NPCs (like commoners, or watch members)

(3) Avoid caltrop use in areas where low level characters may get themselves killed or injured. Remember, caltrops stick around for 24 IG hours or until they deal 25 (I think) damage.

(4) Random caltrop traps are definitely inappropriate

It'd be a real shame to have to remove these items because of inappropriate use.