So, with the Spelljammer book now open again, I've been looking for pre-Spelljammer items and spells that could send people to other planes unintentionally as a concept for a campaign start. I know the Bag of Holding is infamous for dumping everything in and around it into the Astral Sea when ruptured, along with Leomund's Secret Chest scattering the contents across the Ethereal Plane. But is there any other examples folks can think of? (or ways for people to reverse the effects.)

Blink and etherealness spells both involve going to the ethereal. 
Wall of force , temple of the gods and forcecage all extend into the ethereal.

The violet layer of prismatic wall can send creatures to the ethereal.

I’m sure there’s more, but that’s a start. 

Dream of the Blue Veil could be used, if for the 'unintentional' part you either set it up so that the caster thinks they're just granting visions, and isn't aware that it's actually a transportation spell if it goes the full 6 hours. Or you can do a Wizard of Oz and have the entire campaign take place within the 6 hours of the dream, and just have time flow differently because they're in a dream. If the players are supposed to find a way out as part of the campaign, then you have them come up with a way to 'wake up' the caster (who is in the dream with them, maybe in a different location, if that's helpful).

Robe of Stars

Plane Shift

Don't forget the obvious plane shift, though it isn't super easy to use on unwitting creatures.

Looks like Cyb3rM1nd and I had the same thought.

Quote from Xalthu >>

Blink and etherealness spells both involve going to the ethereal. 
Wall of force , temple of the gods and forcecage all extend into the ethereal.

The violet layer of prismatic wall can send creatures to the ethereal.

I’m sure there’s more, but that’s a start. 

 Doesn't blink put you in the Feywild?

Quote from Helmut_McQuack >>

Quote from Xalthu >>

Blink and etherealness spells both involve going to the ethereal. 
Wall of force , temple of the gods and forcecage all extend into the ethereal.

The violet layer of prismatic wall can send creatures to the ethereal.

I’m sure there’s more, but that’s a start. 

 Doesn't blink put you in the Feywild?

 Not the version I was looking at.

https://www.dndbeyond.com/spells/blink

Is there a version that puts you in the feywild?

Quote from Xalthu >>

Quote from Helmut_McQuack >>

Quote from Xalthu >>

Blink and etherealness spells both involve going to the ethereal. 
Wall of force , temple of the gods and forcecage all extend into the ethereal.

The violet layer of prismatic wall can send creatures to the ethereal.

I’m sure there’s more, but that’s a start. 

 Doesn't blink put you in the Feywild?

 Not the version I was looking at.

https://www.dndbeyond.com/spells/blink

Is there a version that puts you in the feywild?

 I was just remembering it incorrectly. My bad, shoulda checked.

Quote from WolfOfTheBees >>

Don't forget the obvious plane shift, though it isn't super easy to use on unwitting creatures.

Looks like Cyb3rM1nd and I had the same thought.

 
Great minds. 👍

🤣

Also, nobody has mentioned astral projection.

Quote from Helmut_McQuack >>

Also, nobody has mentioned astral projection.

I think it's more tricky to think about how this could drop someone unintentionally in the Astral Plane. I guess you could do a narrative "Astral Projection gone wrong" kind of thing.

Quote from TexasDevin >>

Quote from Helmut_McQuack >>

Also, nobody has mentioned astral projection.

I think it's more tricky to think about how this could drop someone unintentionally in the Astral Plane. I guess you could do a narrative "Astral Projection gone wrong" kind of thing.

 Yeah, good point.