pg_xlog filling up under load

Mostly, the disk space used by the WAL files are a constant 80GB, with the files rotating out as new ones are written. This is how I understand it's supposed to work. However, on a couple of occasions, the total disk usage on the pg_xlog volume has grown, and filled up the available space (with the predictable consequences). This seems to have happened under periods of heavy IO load on the underlying disk system.