What is the difference between
バスでスマホを忘れてしまいました
and
バスにスマホを忘れてしまいました?
I vaguely remember there being a nuance difference between these two sentences but I forgot what it was... Please help me
While I think the distinction between で and に in the general sense has been addressed in other questions, I don't think there's any harm in explaining how it works for 忘れる specifically.
バスでスマホを忘れてしまいました
"I forgot my phone on the bus". The で here is emphasizing that the action of forgetting your phone happened on the bus.
バスにスマホを忘れてしまいました
"I left my phone in the bus". The に here is emphasizing that the bus is the place where the action - forgetting your phone - was directed. You forgot something into the bus, if you will, and consequently that's presumably where it still is (unless someone moved it).
To really drive this home, let's look at one more sentence:
バスで席の下にスマホを忘れてしまいました
"I left my phone under my seat on the bus". This sentence is talking about performing the action of leaving your phone under your seat, and that action was performed on the bus. This might be an unnatural level of specificity, but hopefully it drives home the difference between particles.
Anecdotally, I see 忘れる used primarily with で.
Simple answer since this seems to be a possible duplicate...
バスで忘れた focuses on where you did the forgetting (action).
バスに忘れた focuses on where the item was forgotten (location).
'On the bus, I forgot my phone.' vs. 'I forgot (lost) my phone inside the bus.'.
で is where an action is performed and に is the "direction" toward/to/into which the result of an action happens.