Get Everything Done

I have just read through the DIT book and one thing is a bit unclear to me. Say that tomorrow I have only two hours booked for meetings and the rest of the day is "free". Free of course saying I have time to do other work. So tonight I plan my closed list with some calls to make, some papers to write and some emails to reply to. First thing tomrrow I start with one of the meetings and it turns out that the customer is pissed as the products we have installed doesn't work. This our biggest client so it is really importent to have this customer on our side so my prio for today is suddenly to make everything work as I am the consultant. So now I am stuck at the customer all day and even do some overtime to make sure everything is fine. So what do I now do wíth my list for today?