What's a "small" business classic mistake that ended up costing you way more than expected?

Not looking for the obvious stuff like " I should've started sooner"

I mean the little decisions that seemed completely harmless at the time.

Wrong software. Bad hire. Mixing finances. Signing the wrong lease. Ignoring bookkeeping for six months. Spending $3k on a website before getting your first customer.

The kind of mistatke where you look back and think :

"Yeah... that was expensive tuition"

What was yours. and what would you do differently now?