It's even worse for playlists that change (daily mixes, made for you, or curated playlists).

When updating them, Spotify will first delete all old songs and then try to download the new ones. If a lot has changed and you're on a spotty Wifi connection this effectively deletes everything. Happened to me several times while on vacation.

There is the solution of enabling offline-mode before leaving for vacation, but as you mention it's very easy to accidentally disable it and once it's disabled all your playlists are toast.