We have experienced a situation where we are left with large number of thin snapshots (around 500) and snapshot removal wont work anymore (we suspect a lock-up situation caused by trying to remove thin snapshots that is still mounted - thou its not verified yet - just a speculation). The only path to recover seems to be disabling all thin volumes from fstab and booting into single-user mode - where lvremove -f usually succeeds on these thin snapshots. But LVM init and lvremove take a huge amount of time with large number of snapshots - initial LVM activation is going on about 6-8 minutes at least and every lvremove takes ca 20-50 sec - depending on snapshot count left. After snapshots are removed thin volumes can be enabled again and fsck-ed and resuming normal operations.
Althou we are not yet sure what is causing the lockup - so that snapshots cant be removed anymore without reboot - its also problematic that with large numbers of snapshots all LVM device operations are dead slow. I hope this issue can be addressed somehow in the future.