1. Complicated javascript especially ajax-based applications on a site with so many existing plugins and stuffs - where one is blocking another. Then it takes a whole day or more of debugging to find out where the problem is coming from. In this case, we are talking of plugins you downloaded and not codes you wrote yourself. This can be quite disabling atimes especially if the client "must-to" have that effect.

2. PHP Arrays: I am talking of serious multilevel multi-cross-dimensional arrays - actually 5D arrays or more. I had to walk through a 10 level-array yesterday and extract pieces of information at each level. That took me like 12 hours of working back-to-back = and fine i won, and then crashed off to sleep like a log of wood, and still woke up - a little confused.

But of course, these things cant be avoided. . . . . .