Lets see...
I probably have not made my biggest mistake in business yet, as I am still alive, but:
1. Joining the Navy... Put me about 8 years behind my peers, so for a long time it was playing catch up.
2. Ignoring the net for the first couple of years... Didnt get into any heavy web programming at all until 99 or so.
3. Not doing my own thing, instead working for other folks as an employee rather than as an independent.
4. Not following through on my own projects enough.
5. Avoiding being labeled a "salesman" with all my being because I couldn't stand the folks I knew with that label... Sales is the driving force behind all business and always will be.
damn, there are so many