Reminds me of a bug in PostSharp.. After making some changes our app would throw a NullReferenceException Mondays. It turned out to be a bug in some optimisation, but it only occurred on Monday because they skipped licence checks unlocking "full functionality" on a Monday (Happy Monday!) which enabled the optimisation.
https://plus.google.com/113181962167438638669/posts/QF5pDB4X...