Spurious failure to obtain row lock possible in PG 9.1?

I'm "pretty" sure there's really no other process that has the lock, as I'm the only one on a test DB. If I execute the query immediately again, it does succeed in obtaining the lock. I can however not reproduce this via e.g. PGAdmin.

Is it possible or perhaps even known that PG has this behavior, or should I look for the cause in the Java code? (I'm using Java EE"s entity manager to execute a native query inside an EJB bean that lets a JDBC connection from a pool join a JTA transaction.)

Thanks!