Loads of mistakes...
Selling talismans I thought I didn't need like basic health, strength, and holy - then having to go farm them. I also levelled cyclone, resistance, resilience and explosive, although now that doesn't look too bad. Wish I'd kept my thorn set.
Wasting ironite on XP boost
Wasting ironite buying gold
Not getting a good full list of troopers for months - used to only have about 10 ages after I started.
Wasting gold and runes using evo runes to level up talismans, sometimes with the wrong colour (not an issue since the drop rates were fixed but for a while it was almost impossible to level talismans)
Putting skill shards on Basic Eddie and Wickerman Eddie.
Farming Game-of-Death on Madness, then hitting 'next' instead of 'replay' and ending up facing the Beast with the wrong team (only happened twice tbh)
More recently: Only ever buying one set of Immunity and Shell talismans whilst they were available for like a month - not like I didnt have the iron coins.