A colleague of mine recently told me that he once wrote a web application that made very heavy use of StringBuffer class (JVM version 1.3) in order to generate code (using Javacc) and experienced the problem of running out of file handles.
It was explained to me that the problem was due to the synchronized methods of the StringBuffer class and all the thread locking associated with using it.
I don't know much about this but it sounds very interesting and could explain a problem I had at a previous company.
You might ask your colleague for clarification, because either you misunderstood what s/he was trying to say, or, well, s/he did. I'm certainly not aware of any OS that uses file handles as thread locks.