Serena not getting #24 and #25 is more bad luck than anything. Many times, she just ran into the hot player, like Naomi Osaka and Bianca Andreescu. Other times, it was injuries like the 2020 French Open. That's just part of the game, nothing you can really do about those.
That said, the one I'm really angry about is 2019 Wimbledon. Simona cheated, won, and it caused a trickle down effect. It cost Serena the traditional opening slot that is given to the defending champion at 2021 Wimbledon (it went to Ash Barty), which caused her to play in the slippery conditions that contributed to her injury. Serena had a forgettable ending to her career, while Barty ended her career on a high note, winning 2021 Wimbledon and the 2022 AO. I know nothing is guaranteed, but I wonder what if Serena (rightfully) got the traditional champion's opening slot, and Barty had to play in the conditions Serena played in that day.
Makes me mad knowing that if not for Simona's cheating, Serena would have won the 24th slam at 2019 Wimbledon, and possibly 25 or even 26.