I cannot define the biggest mistakes as it would be too fundamentalist for me but I guess one of my biggest "mistakes" was underestimating the lengths that criminally minded people will go to in order make money
Biggest mistakes in my life: 1. thinking I know everything. (didn't take long for that one to be squelched) 2. trusting people without them earning it 3. feeling guilty about my mistakes instead of learning from them. (guilt is useless)
As a whole, I think the biggest mistakes people make are to live their lives unhappy.