I'm a little pissed off with myself.
I have a credit card, that I haven't got round to setting up a direct debit to pay it off automatically in full every month yet, so I've been paying it manually.
For November 2016, the balance on my card was £1355.46. Unfortunately when I paid it off, I went all number-dyslexic and actually paid off £1335.46 (£20 short).
That then meant, that because my card wasn't paid off in full that month, they could charge me interest on all of my purchases. So, for the sake of being £20 short, I got charged £19.33 interest for the month :(
The next month, I was going home for Christmas and I had to leave before my credit card statement arrived. No problem - I can just log into the website to check my balance. I do this, and schedule a payment for £317.12 to cover that month's purchases so far, and resolve not to use the credit card again in the next week or so.
Great. However, at that point I didn't know about the £19.33 interest I was going to get charged from the last month. So, my payment ended up being £19.33 short of paying off the card in full - so I get charged another £5.72 interest the next month!
Dammit, what a waste of money. I really should set up that direct debit...