The bug

When a player joins a world or server, repeating command blocks begin to run before they are fully loaded into the game.

What I expected to happen was...

I expected for the repeating commands blocks to only execute when a player is fully loaded, so that they doesn't falsely execute a successful testfor command.

What actually happened was...

A repeating command block checking for a player will fail momentarily, even though the player has joined.

How to reproduce

1. On singleplayer, enter the following command in a repeating command block:

   execute unless entity @a run setblock ~ ~1 ~ stone

2. Exit and rejoin the world
   →  A block of stone has been placed above the command block

Original steps to Reproduce

1. Create a new world of any type in creative mode.
2. Place a repeating, no-redstone, non-conditional command block with the command testfor @a.
3. In front of that, place a redstone comparator, a solid block, and a redstone torch (on the block).
4. Adjacent to the redstone torch, place a repeating, redstone-reliant, non-conditional command block with a say command (e.g. say eggplant).
5. Nothing should happen; this means that the first command block's testfor @a command is successful, preventing the second command block's command from executing.
6. Exit and rejoin the world. [@] eggplant will be in chat twice, meaning that the command blocks executed for about two ticks before realizing the player existed.

This is a problem if you have a message that shows up when nobody is on a certain team, or if you want something to run when there are no players online (in a server).