Screwed by the Network: Happened to WWE Saturday Morning Slam, as The CW wanted to change the format of the show to a WWE news magazine rather than the taped matches that the company wanted. WWE didn't want the change, and thus canceled the show.
The Muhammad Hassan angle on SmackDown and Who Killed Mr. McMahon? Although both were more of "too early" examples. The first was filmed before the July 7th bombings, it was only aired after them. The latter angle was actually stopped because of another incident to which the WWE performed this trope: the Chris Benoit double murder-suicide. When the news first broke of the tragedy (before anyone became aware of the circumstances involving the deaths), the WWE dedicated the episode (filmed in an empty arena) to the life and career of Benoit. However, the very next day, it was made known that Benoit killed his son and wife before killing himself, and thus, on the ECW show, McMahon mentioned that Benoit's name wouldn't be mentioned at all on that show, and that the wrestlers would "do what they do best. Entertain you!" Interesting to note that the WWE has yet to acknowledge Benoit's existence since that night, to mixed reaction, and made a literal Hand Wave of his matches and appearances in their back library since then.
What Could Have Been: With arcs getting aborted mid through, either due to injury of one of the wrestlers involved, lack of reaction or other issues there are some things you really wanted to see happen after reading the plans.