When you launch the rocket, you are prompted to save the replay. In multiplayer, in the save dialog that pops up, the normal functions of the keys are not disabled while typing here. So if you write a save name that includes one of the letters "wasd", your character will move around on screen while you type. This does not happen if you enter the save dialog via the normal escape-menu.
Also, me finding this bug is because I have launched too many speedrun rockets...
[0.14.21] Writing replay name in multiplayer moves player
Re: [0.14.21] Writing replay name in multiplayer moves player
Fixed for 0.15.
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