This was a reference to Halsey's actions at Leyte Gulf where he "swallowed the bait" and attacked Ozawa's mobile force of Japan's surviving carriers. This left the Leyte landing unprotected by the strongest American naval force in the area. This decision has remained controversial among within the Navy andnaval historians ever since and Clancy's reference to it reflects this debate.

The response from /u/kieslowskifan may be correct, as that is Halsey's best-known mistake.

There are also references in Clancy's book to Ryan writing a book regarding Halsey's actions during Typhoon Cobra in 1944. Three destroyers were lost and many ships were damaged. I don't recall the exact excerpt from the text of Clancy's book, but this reference could also be the Soviet captain making it clear that he had read Ryan's book. That's what I recall, anyway.

It's not a great source, but the final item in wikipedia's entry on mentions of Halsey in popular culture agrees with my recollection.

I found this online, and it appears to be the text in question:

Capt. Vasili Borodin: Torpedo impact, 20 seconds. Captain Ramius: [to Ryan] What books? Jack Ryan: Pardon me? Captain Ramius: What books did you write? Jack Ryan: I wrote a biography of, of Admiral Halsey, called "The Fighting Sailor", about, uh, naval combat tactics... Captain Ramius: I know this book! Capt. Vasili Borodin: Torpedo impact... Captain Ramius: Your conclusions were all wrong, Ryan... Capt. Vasili Borodin: ...10 seconds. Captain Ramius: ...Halsey acted stupidly.

I don't know if I can say for sure which mistake the captain is referring to, but it seems to me it would either be going after the carriers or sailing into the typhoon. I know of no other major errors on Halsey's part that Ryan (via Clancy) would be referring to.

Side question: if Halsey's reputation for aggression to the point of recklessness was already established by May 1942, why did he suggest Raymond Spruance (regarded as extremely conservative) to take over operations at Midway?