Selling after the Mt. Gox drop + not buying in for 2 years and forgetting about halving 

The biggest mistake I've done when the first time i read about bitcoin i didn't believe on it that is the time that bitcoin price is only 1$ i regret in my life that why i don't believe to it.  The biggest mistake that never forget if i recall everything i dream that am a millionaire now when i buy bitcoin that time.

It seems mistakes over here are unavoidable and it will even be surprising that the same mistakes put on record here will still be made by someone else in the near future when there is a platform for record keeping like this. In my own case, my mistake was trusting a ponzi scheme after reading that at the beginning they pay then scam later. So, I joined the site at the very time it begins and to my surprise as at the time for me to make withdrawal like 3 days later, the site has gone scam and that was how my money went down the drain.

My biggest mistake was not getting into mining or at least mine on my PC back when I first ran across Bitcoin miner early.
But I started mining at late 2014. After that I read some article saying it could hurt my PC so I just stopped mining. And my PC  was attacked by some silent mining program and i was unable to recover my PC.
This is my biggest mistake ever i do.

Before two year I need to send $15 for giftcard guy sent me a message with different address aand I sent him money then again sent me a message that send money to this address first one was wrong address by mistake and said previously sent money will be refunded to you as no one owns that address because 0 transaction and then after I've sent money no response from him.

This happens when I first used bitcoins and the seller of giftcard only accepting btc. I learned a lot from it.

As a newbie I was not much aware of wallet and the features along with the same. So the weaker security made me lose small portion of my bought bitcoin.

We did the same mistake i do really strongly believe on that.It cant really be avoided since as a human we do really panic when we do see the price of bitcoin goes down and we would really panic specially when we dont have enough knowledge on bitcoins movements and other stuffs and as you said regretting would really happen on the end but still we do learn on those mistakes.

i dont have a terrible story to tell.
but selling those bitcoins at the price which is much lower than it is now. this is the biggest mistake i have done till now.

These are good words for those who still believe that thanks to gambling you can get rich or win a lot of bitcoins. It's not true. The user still for all the time that he plays gambling, spends a lot more than he wins.

There are many mistakes I experienced in using bitcoins but there is one is unforgetable for me. When I was starting in bitcoin I used to gamble to gain earnings. When I have a capital of 0.00005, I used it and then grow for about 0.001 and then when I got sleep I didn't recognized that I was lose real quick. And then when I play gambling I sure that I won't lose again.