Custom beehive item can be used to spawn any entity, including command blocks

 Custom beehive items can be used to summon any entity, including command blocks. They're easy to smuggle in as long as you have creative mode on a server, thanks to the saved toolbars feature.

Steps to reproduce:

1. Go into a single player creative world and give yourself a special beehive:

   give @p minecraft:bee_hive{BlockEntityTag: {Bees: [{MinOccupationTicks: 2400, TicksInHive: 2280, EntityData: {id: "minecraft:bee", Passengers: [{Command: "say This command could be anything", id: "minecraft:command_block_minecart"}]}}]}}
   

2. Store the beehive item to your saved toolbars.
3. Join a server where you're not an operator but have creative mode.
4. Retrieve the beehive from your saved toolbars, place it and wait for the bee to spawn (the command block minecart is there, but in an invisible broken state).
5. Cause the server to reload the chunk with the bee from disk (restarting the server always works).
6. Congrats, you have created a command block minecart with an arbitrary command inside without being an operator.