What's the dumbest mistake you have made in the game that you realized too late?

I just smacked myself in the face because I was getting extremely frustrated that I couldn't catch a Pufferfish. I spent the last 6 days of the season, I spent a lot of money I barely had on tackles and deluxe bait. I triple checked on Wiki if it was indeed Summer Ocean between Noon and 4PM on sunny days and it was.
Then it finally hit me. I was still in Spring. I wanted to spend those days grinding for gold so I could get quality sprinklers for the next season. Now I'm struggling to do the math on how many melons I can afford ;-(