If the player count is one below the maximum and 2 players join at once, they are both let in and won't get kicked, resulting in there being more players online than the maximum allowed.

To reproduce:

1. Set up a fresh server and set "max-players" in server.properties to 2. (Note: In my testing I also set "offline-mode" to false since I don't have 3 accounts)
2. Boot up 3 seperate accounts.
3. Join with one of them
4. Now you have to quickly join with the other 2 accounts. To best reproduce this I recommend to use a tool like clumsy and set your ping to 100ms. Alternatively you can try to join with all 3 accounts at once to increase the chance of it happening.
5. Now there are 3 players on a server with a player limit of 2

This does not work in vanilla.