Last night, I DMed a game with only two players, and neither of them had any prior experience with D&D. All in all, it went great, but towards the end, they got into some serious trouble and the druid seemed somewhat surprised by the fact that she had used all of her spell slots. I didn't want to punish a beginner too hard and the only alternatives appeared to be a near inevitable TPK or some cheesy deus ex machina, so I told her she could try to cast Healing Word despite having used all of her magic power for the day. I let her make a CON save to decide how she could handle the enormous stress of stretching her abilities to such an extend. She rolled quite high, but not extra-ordinarily high, so I decided that she could indeed successfully cast the spell, but that it might backfire later in some way. I haven't decided the specifics yet and in order to keep it interesting but balanced, I am looking for something similar to this in any official source book.
I'm aware that I'm well into homebrew territory with that ruling, and that this is not the right site to ask for inspiration. This is why I am specifically asking the following:
Is there any class or racial feature or any item that allows a character with no remaining spell slots to cast a spell of level 1 or higher at the cost of some negative consequence (e.g. taking a level of exhaustion)?
I am not asking for general ways to simply cast spells without expending a spell slot. There has to be some immediate trade-off. Taking 18 wizard levels in order to gain access to Spell Mastery can of course be seen as quite a trade-off, but I hope it is obvious that this is not what I am looking for.