When Shawn Kemp and Gary Payton were traded. As much has it hurt, it sorta made sense to trade GP and without him leaving, he wouldn't have got his ring (Thanks to Shaq and Wade) but Kemp being traded was utterly stupid. The Sonics should have paid Kemp to keep him happy. We could have made multiple trips to the Finals and win at least one. Kemp would have not gotten fat and while it's a long shot, the Sonics could have still been in Seattle.
Speaking of which, Sonics leaving Seattle while rather recent really sucked. I've made my peace with it but going to Key Arena now isn't the same.
Griffey being traded also hurt. A year later, Ichiro came to the M's. When they were on the M's in 09 that flip my shit. But I can't help but imagine how the team would have been with Ichiro and Griffey in 01. Maybe we wouldn't have had that 116 win season. But perhaps we would have made more playoff appearances. Ichiro only have 1 playoff appearance is madness to me.