ora-29861

Can someone explain ORA-29861 error in plain english and its possible cause?

I have an application implemented in Grails framework using underlying Hibernate. After it runs for a while, I got an Oracle DB error and resolved it by rebuilding the offending index. I wonder if anyone can propose the possible cause(s) and ways to prevent it from happening. Caused by: org.springframework.jdbc.UncategorizedSQLExce...