I released a new version of Cake's Huge Opening Book (CHOB) recently. The new version is now based on about 2.35 million positions that were analyzed by Cake, the previous version (released in mid-April) was based on about 2.2 million positions. The new book would be approximately one ply deeper in the shallower main lines than the old book; however, I also threw out all positions which are in the leaf of my book tree - i.e. those positions where no successors have been searched. The reason I threw out those positions is that they are based on a 90-second book-generating search on a fast computer (AMD Athlon 64 3400+ with 2GB ram), which is good, but perhaps not good enough. The book-generating search produces evaluations for all moves in a given position, and takes longer than the regular search to reach a specified search depth. Therefore, it is approximately equivalent to a 30-second regular search on a fast machine. That means that if you set Cake to search for more than 30 seconds on a fast machine (and if you have the 8-piece database), then you might get a better move than what is in the book. This alone is enough to throw out the leaf positions, but there is more: Ed Gilbert actually found a losing book move in my first release of CHOB, and it was just such a leaf node. Now, this move is no longer in the book, and if you give Cake enough time, it won't make this losing move. Looking at the data from the opening book generator, it seems that it will have to run for another 2-3 months until it will expand that position with the losing move further.
Having found (or been told about, rather) one error in CHOB 1.0, I have to assume that there were more errors. After all, Even in a 288-game-match only about 288x10 = ~3000 book moves are actually played by Cake. Now that I know that one of these was a loser, I could extrapolate and say that 1 of 3000 moves is bad, and that therefore there are approximately 1000 losing moves in the book. I don't think it's this bad at all though, but there is no way to really know.