First make sure you’re collaboratively editing the same document.
Preferably on different positions in a very long (and unbelievably ugly coded) file.
Let the fun begin:

1. Open up a multi line comment and never close it.
2. Repeatedly press Cmd+A to highlight _everything_ so the screen for all the others starts to flash in your editing color
3. Simply paste in A LOT of blank lines so the functions, the others are currently working on, scroll out of their view
4. Sneak in some nasty syntax errors and wait for someone to save the file to test their newest edits

Some more ideas? 🙂