I set the priority as Critical because of potentially destructive consequences. Except the fact that the thread which was trying to save the entity hangs, it is also running SELECT and UPDATE queries on HIBERNATE_SEQUENCES table all the time. In many databases with default isolation level this would result in table or row range locks on that table. Which means that not just this thread, but other threads trying to generate IDs for other entities will hang. The effect accumulates over time and can lead to all application server working threads waiting for the first thread, causing the application to stop responding.
The error however is specific to this generator type, and has an easy workaround, so feel free to decrease priority if necessary.