For the many years I am now practicing GTD® the one thing I have learned is to never underestimate ad-hoc work. There are times where ad-hoc work is the default situation: My company has just recently finished its fiscal year and as a sales manager it was evident that I will be bombarded with ad-hoc issues and requests in the last 2-3 weeks leading up to the event. As a consequence I simply did not plan much, some days not even any, scheduled work (predefined tasks on a todo lists). Instead I deliberately sat in front of my email inbox, instant messenger and phones and waited for the work to show up. And it did show up like a truck rolling over me.
I’ve never thought of a good way to describe the work you never planned on doing, but “ad-hoc work” sounds pretty good.