What was your biggest sacrefice/mistake in WoT?

Ectar: Selling the Centurion 7/1 3 times before understanding how to play it. Selling my Pershing to fund the purchase of my Centurion. (Centurion is a great tank, I just also miss my Pershing) Not taking more an active interest sooner in my stats with a view to improving them. (Damage done & accuracy mainly which naturally lead to kills and wins)