Episode 28 of Rurouni Kenshin introduced Saitou Hajime with a bang and officially kicked off one of the series' pivotal arcs. This is more obvious in the manga, which changed from small arcs barely connected to each other to large arcs that follow one another closely; the "Wham" aspect, however, is more pronounced in the anime, as it had just come off a string of lackluster filler episodes, making the coming episode a real shocker (which was even reflected in the previous episode's more ominous Post-Episode Trailer).
The anime makes a real attempt to distinguish this episode from previous ones stylistically, as well. It has a much darker, more slow and methodical atmosphere than other episodes, the backdrops change to being either at night or in perpetual cloudy gloom (compare to the sunny skies of previous episodes) and even the music consists mostly of new tracks. They also change the ending theme song. It does a very good job of sending the message that things are about to get very serious.
The arc which follows has quite a few Wham episodes of its own, such as Kenshin's sword being broken.
The second Wham occurs when the restaurant Kenshin frequents is destroyed, and Enishi announces his Jinchuu on Kenshin, changing the manga from the previous stereotypical shonen battle, to a more personal series.