I've seen a hang when dropping an LSM tree. The relevant stack traces are:

sched_yield,__wt_lsm_tree_drop,__wt_schema_drop,__drop_table,__wt_schema_drop,__session_drop
__lll_lock_wait,_L_lock_816,pthread_mutex_lock,__wt_session_get_btree,__lsm_drop_file,__wt_lsm_free_chunks,__lsm_worker,start_thread,

This could be related to WT-1380 since I'm running on that branch. To produce the hang I populated an LSM tree, and then did a drop on that tree a short time after (not long enough for the tree to settle).