OK, this was just annoying, but since I couldn't find anything on the web when I looked, here's what my problem was:
I was modifying some hibernate code to use the SequenceStyleGenerator annotation, so it will work cross-DB. I'd initially developed using MySQL, and had a MySQL hibernate dialect specified in my config files. I exported the schema to an Oracle XE instance using hbm2ddl in Eclipse, and started using an Oracle driver in my datasource.
Every time I tried to insert anything, I got an oracle error - Sequence not allowed here. Ultimately, just removing the dialect entirely from the config files worked, since Hibernate can auto-detect the dialect most of the time.