I definitely share this one, although for me it was taking out a too big student overdraft while it was interest free & then not paying it off before the interest free bit went away 
I wish I’d recognised the value of zero interest balance transfers for credit cards sooner too!
Just to mix it up a bit, one ‘waste of time’ that I don’t regret was ‘investing’ in gold in 2012. I held it for a year & then sold at break even. I didn’t make any money but I learnt a lot from the experience.