When it comes to spells in D&D 5e, I had a very good overview. It's not like with subraces or feats, for example, which are scattered across a multitude of official and less official sources. The number of spells is quite manageable, as they are all contained in the Player's Handbook, the Sword Coast Adventurer's Guide, the Elemental Evil Player's Companion, Xanathar's Guide to Everything, and that one Unearthed Arcana article (UA: Starter Spells).
...Or so I thought.
Then I stumbled over Flock of Familiars, a 2nd-level conjuration spell that lets you summon three familiars at once. This spell appears in Lost Laboratory of Kwalish, an adventure officially published by WotC.
That made me wonder:
Which spells are hidden in adventures and other sources?