Many actions in WordPress will fire multiple times, which can lead to performance drags and undesired/intuitiveness results. Pippin Williamson points out that you can check how many times an action has already run, and modify your code to respond accordingly.
Jeff Atwood wrote a fascinating article on different methods to use to discourage trolling on social websites, including some discussion of their ethical merits. I wasn’t aware of slowbanning and hellbanning before, but I think they’re very clever ideas.
It’s often much more efficient to access DOM properties directly, rather than using jQuery. The downside to that approach is that there are differences between the various browsers, so in some cases it introduces bigger problems than the extra performance overhead.