[throwaway2016a]

This is one of my "favorite". I often remind junior developers who just started coding in the last few years about stuff like this since a lot of our client's still insist on IE8 support (thankfully not IE 6 or 7 anymore).

Another fun one was dangling commas in arrays, which breaks IE 7 and lower.

It used to be somewhat common to put something like this at the top of scripts:

if ( window.console == undefined ) { window.console = { log: function() { }; }