OMG - The Wheel of Time series. One of the biggest reading mistakes I ever made... like most sane people I had thought it would be maybe four or five books at the most. I plodded on with the series as it got longer and longer, getting increasingly frustrated with it, until it got to a point where to remember WTF was actually going on in the books I had to re-read the preceding ones in order to read a new one and it all became a completely pointless waste of my time.
The mass of books just became a burden on my bookshelf and I couldn't sell the bloody things - in the end I even trouble giving them away.