Most recent: I was making 2 blankets.

I go to the store (2 hour round trip, I don't want to wait 2 days for postage), buy the wool and go home. Get home, realise I bought the wrong size and have to go back in the morning. So another 2 hour round trip. Exchange items and pay difference, but they don't have enough in stock. That's ok! I'll buy more online!

Forget to buy more.

Complete first one, start second, almost at end of wool. Realise there won't be enough. Remember I needed more wool. I don't remember how much more wool I needed. The receipt has faded completly. Eye up progress and guesstimate how much more I need. Add extra for safety.

Wool is not available in my country anymore in the right dye lot. Have to buy online. Only option is UK. Buy wool from multiple sources, one has to be shipped twice as the store won't ship outside the UK.

Finally finish. Have 16 extra balls of wool.

Cost: extra 110 euro. Almost 2 hours of time, petrol money. And 16 balls of wool to add to the ever growing stash of wool I keep trying to reduce but somehow keeps multiplying and expanding steadily.

Because I was too impatient to wait for shipping. Impulsively went with the size suggested by the clerk as it sounded right and didn't double check. Forgot to keep track of the wool I had and used. Dropped the project and didn't pick it up for months, meaning the receipt was faded by the time I needed it and had paid in cash as I can never remember my pin so no record. Intended to and forgot to buy the extra wool when it was more easily available and cheaper to source.