三人行，必有我师(焉)

在家, at home

I always do a double-take on this one, because more than once I've been very confused by finding that the Wiktionary entry for 在家 says it means both "at home" and "at work." XD Just now I went to check MBDG and got a somewhat better explanation there: "(at a workplace) to be in (as opposed to being away on official business 出差)."

I'm still not sure if 在家 only means "not away from the office on official business," or if it means "in the office" more generally (as opposed to being anywhere else, like away on vacation, out on sick leave, gone home for the night, etc.)... but I presume not, since if the person had gone/stayed home they would... actually be 在家???