This is a bunch of rough-ish thoughts on the mistakes I've either avoided or have made and learned from. These ideas may or may not apply to you, so use your judgement.
An instant messager.
This is the notion of "freezing" Windows in time, such that it can work as-expected but be "rolled back" to a previous state.
I'm also lumping similar projects into this page, for reference.
See also Snapshot (computer storage)