Here are some crashes with nested liblmdb transactions.
Fixed in mdb.master, except:
Can't use a DBI from an aborted child txn, unlike aborted main
$ ./bug R[cp] # Create a named DB and put an item there
$ ./bug [[o}g] # Open that DB in a child, abort, use the DBI
40: mdb_get(txn, dbi, &key, &rdata): Invalid argument
Fixing this caused problems and got reverted.
The DBI is available to next non-nested txn:
$ ./bug [[o}}[g] # Open DB in child, abort both txns, use DBI in new txn