When enhanced table-based ID generator (org.hibernate.id.enhanced.TableGenerator) is used, it is generating new IDs based on a database table, named HIBERNATE_SEQUENCES by default. The NEXT_VAL column in this table is generated as nullable. If it contains a NULL value however the ID generation hangs and never completes, generator code is running SELECT and UPDATE queries in an infinite loop.
A test case is attached, it should only require Hibernate and HSQLDB to run.