Wikimedia Bugzilla is closed!

Start the visual editor on wikimedia.org. Now hit several character keys simultaneously; often you will see the characters inserted plus a spurious newline. 

It doesn't always happen; most consistently, I can create the effect if I place the cursor at the beginning of an empty line that has several other empty lines underneath. I then use four fingers to hit the keys jkl; simultaneously. 

The effect does not depend on the specific keys hit; it's crucial that keys are hit simultaneously or nearly simultaneously. It's possible but more difficult to elicit the effect by only hitting two keys simultaneously. Strange effects can also sometimes be seen if the following lines are not empty, but they seem to be rarer.

I see the effect in Firefox and Chrome on Windows 7. Since it appears to be a race condition, the speed of the computer probably plays a role, but I haven't investigated.