In the Star Trek: Voyager episode "Before and After", Kes shifts backwards through time. Part of this time is spent during the "Year of Hell", during which she learns vital information about the Krenim weapons. She gives this information to Janeway.
In Season 4, we see the actual Year of Hell. Kes has left Voyager by this point, but the information she gave Janeway is still valid. Yet Voyager suffers a great deal of damage and nothing seems to have changed.
The Year of Hell episodes were even advertised as sequels to Before and After, so they definitely hadn't forgotten the episode.
It's like everyone forgot what Kes said, or ignored it. Does anybody know why Kes' warning was ignored? Canon or behind-the-scenes references preferred.