Probably related to others similar topics; this won’t happen often but one way to replicate the bug is to prepare let’s say 10 pickets, pause the game, place them and resume at max speed (not sure it the speed is involved, but…).
Eventually an Hearthling will drop the picket correctly but an iconized version of the later is generated and fall on the ground. If an Hearthling pick it up (to put it back in storage), he will freeze and then adios amigo!
Not sure if the replication of the iconized version of the item can happen with other buiding mats. More feedback it detected.