My biggest mistake was a couple of years ago and revolved around greed and not wanting to give up control.
Had a small online business that became a medium sized online business - but I kept running it solo. Ended up doing about 200K/mo in revenue and let customer service fall by the wayside.
Retrospectively I should have employed 2-3 staffers at 35K/yr and focused on business development rather than trying to save the 70-100K/yr.
However, that lesson has helped. I now bring staff on early and get them to specialize. This means better results, faster and more reliably.
It's all right to make mistakes, as long as you learn from them.
-aReJay