What is your biggest regret (purchase or non-purchase)?

My one regret miss was an item that came out 3 times and I missed it every time.

The Duff racer. I still want it


I also regret purchasing the Krusty Monorail station because I really only got it for the 10 tracks. Now that I really don't have plans to make a larger monorail system and plan to keep that station in my inventory, it was a big mistake