Spend months wondering why something happened the way it did when it got written months ago, then *ping* the proverbial lightbulb, and then *whoosh* the whole history of who and what and why and when and where and we're suddenly discovering the O'Brien family - Molly Weasley's side of the family - and life in the 40s and 50s in Ireland. See molly_mum.
Time to go and do more background reading... after more drugs.