WWE SummerSlam was the stage for several major championship matches and after the crowd had already seen Bron Breakker take the Intercontinental Championship from Sami Zayn, they were treated to another title change.
LA Knight pinned Logan Paul following a BFT, but the issue was that his foot was underneath the ropes.
Clearly, neither man looked that way and had the ring awareness to notice that he was under the ropes, but this isn't considered a legal pin. Often wrestlers will drag their opponents away from the ropes for the pin because anything outside of the ropes can only be used in a Falls Count Anywhere match.
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WWE has enforced this rule several times in the past most notably back at Roadblock in 2016 when Dean Ambrose had Triple H beaten for the World Championship but because his foot was outside the ropes, it didn't count.
This could be part of the storyline and could lead to Knight being forced into a rematch at Bash in Berlin, or could have been a genuine botch and neither man was aware of it until Paul's foot landed on the ropes following the pinfall. The referee also didn't look in that direction to see his leg positioning.