debugger is a nice keyword, although I've noticed it slipping into committed code (at work) more and more frequently. Our code review process catches it most of the time, but it's noise in the source control system (another commit to remove it).

Doesn't make it any less valuable, and a stray debugger; statement doesn't cause any problems out in production.

The last three were new to me. Emulating mobile layouts will save a heap of time, currently I use Electric Plum's simulators (which are good, but another tool to set up).