The fatal variance defense is a rare but effective defense strategy for attorneys. A fatal variance is when specific acts alleged in the indictment are proven to have actually occurred in a different manner.
A common example of a fatal variance is when a prosecutor alleges that a defendant stole checks, when in fact he stole currency. The purpose of the rule is to put defendants on fair notice to prepare their defense, which would be much harder to do if a prosecutor could bring up random charges.
So if an indictment alleges that a defendant spoke specific words, and then a wiretap proves otherwise, the defense can assert that this is a fatal variance from the indictment and make a motion to the dismiss the case.
More often than not, it will be a successful motion.