On many lines, bus service stops before metro service does. Some of it stops ridiculously early, e.g. the King Street trolley in Alexandria stops running at 10:15 p.m. even on weekends (though it isn't a bad walk along King Street to the metro).

Some of the Circulator routes run to midnight (till 2 a.m. on weeknights), but some stop at 9 p.m., including Georgetown-Union Station.

The other thing to watch out for re: pbulic transit here is weekend and evening track work. There is a rew week respite right now for the Cherry Blossom Festival, but weekends often mean closed stations and lengthy delays most of the year and especially on holiday weekends.