When you find a seeming inconsistency in the rules of math, or logic, or probability theory, you might do well to consider that math has rightfully earned a bit more credibility than that. Check the proof. It is more likely that you have made a mistake in algebra, than that you have just discovered a fatal flaw in math itself.
Discuss the post here (rather than in the comments to the original post).
This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Beautiful Probability, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.