Two users getting procmail output error

I have my mail server working pretty well except for a couple of users. When mail is processed for them I get Command output: Procmail: Command failure (65) and/or (75) Although the message says it is temporarliy deferred, eventually the mail is bounced. I cannot see anything different, with respect their mailboxes (ownership, permissions, etc.), from everyone else that is working. What do these errors mean and how can I debug them? TIA.