My pet peeve is having my computer underline the word "outwith" or, worse still, having someone wrongly correct me when I use it.

It's a perfectly nice little preposition that doesn't have a direct synonym but does have an uncontroversial antonym, it can be found in the Collins, Cambridge, Oxford, Merriam-Webster dictionaries, it is used in the legal systems of several countries, it is recorded in Hansard (official transcripts of debates in the UK Houses of Parliament) on countless occasions, and yet many people and most spell-checkers refuse to acknowledge its existence as a word.