Fairly regularly we get people reporting errors about their watchlist not loading. Usually its because they have very large watchlists (10s of thousands of pages).
(Cf. T41510: Opening Special:EditWatchlist with a large watchlist hits server timeout (Create watchlist pager) and the other tasks mentioned there)
We should have some documentation (wikitech) for how users should try and resolve this; including clearing their watchlist themselves via Special:EditWatchlist, Special:EditWatchlist/raw or Special:EditWatchlist/clear. And in extreme circumstances, it being done server side by someone with shell access.
We should also include information on how users can help prevent this in future. This includes turning off Watchlist preferences such as Add pages and files I edit to my watchlist and Add pages I create and files I upload to my watchlist; this is especially relevant on wikis such as Wikidata where a user may create or edit many items, and not actually be fussed about watching them in future