I've spent more time helping people hunt down "undefined is not a function" and "Cannot read property of undefined" than just about anything else. Two observations from this:

1.) The fact that I even got involved meant the issues were particularly hard to track down

2.) These errors seem to be more of a time sync within a larger team/project than many realize

Typically these were caused by either scope issues and/or some context object that's been riding dirty all day. In the case of the latter hunting down the code that did the naughty can be particularly soul crushing.