to read up documents (1000+) and then worked with them. The problem I ran into was that if the users session had expired and the code still ran but had no access to the documents any more.

The server then crashed with a "allochandle: out of private handles" filling up the console.
Apparently - even though I recycle() - trying to do operations on a document you do not have access to uses up private handles.

The solution was to test the doc.isValid() before attempting to use the document - and of course to recycle().