Fortnite's new patch dropped today, but it looks as though fans are struggling to board the Battle Bus and get playing.
A new "That Wasn't Supposed To Happen" error is preventing players from logging in.

Fortnite Down: How To Fix "Battle Buses Are Full" Error Code
Check out the screenshot below, with the error code "match.session_unavailable.402".

Uh oh
Luckily, it seems the issue is easy to fix - just make sure your game is updated.
Here's how to do that on all platforms:
PC
Quit Fortnite, and if an update is available, it'll be downloaded via the Epic Games Store.
PS4/PS5
On Sony consoles, head to your dashboard and highlight Fortnite.
Press the options button, and then "Check for update".
Xbox One/Series S/Series X
On Xbox, head into "My Games and Apps".
From there, check the "Updates" section on the left, and ensure if Fortnite appears that you install it.