An exercise like this could be used to reinforce and build intuitive grasp of levels of abstraction: Tell people to get a candle and light it. Then they have to write fifty things that describe that candle. They will likely start struggling after a while. Then you could give them a hint – “Give your reader enough information to understand your experience“. They would have to include details – get more descriptive. But they cannot (as of 2015) transfer the experience itself.
If you can’t reproduce a certain bug, study only that part of the system so that you can get it to fail. If it’s a problem with the session, study the class(es) dealing with handling of the session. See what methods there are – is the session getting cached in the db? Is it getting cleared from $_SESSION in an attempt to save space? Is there anything in there that looks like it might be related to your bug? Studying those particular aspects of the system will hopefully give you enough understanding so that you can make the bug reproducible.