The worst thing is that i am just repeating silly mistakes and just few days back i ended up with massive 650 USD losses, it’s just not learning from the mistakes which makes me extremely frustrated and worst thing is you know that you are making mistake yet just due to been greedy you continue on and on! It’s just about avoiding greed that can save me from disasters!
A couple of times (some years back) I left live trades open without a stop loss. I left the house to go do things. I am lucky not to have lost much more money over the years.
I did mistakes due to my low knowledge and experience . after knowing facts about forex trading I mostly try to do trading with rules plan and proper preparation. I never make high targets that are impossible or put my account in danger. All of us learn from mistakes , it is good to improve our trading not to stuck on any point where we can not adjust trades according to market.
While I was looking after my 2 1/2 yo I placed a trade. My confidence was sky high after many successful scalping trades. I thought I had this trading thing sussed!!! DOH…about 2 hours later and after desperate attempts at trying to recoup my initial loss I walked away from the computer after copping a 40% loss - OUCH!! I was trying to trade without a SL as a friend of mine does and opened a large position. I had been trading live for about 6 months (demo for a couple of years) and it’s been about 3 months since that 40% hit. To be honest, my confidence is not what it was, but I’m not a quitter! I’ll stick at it because I love it and intend on trading full time one day 
I learnt 2 valuable lessons that day - 1) check for any news (I check forexfactory) and ALWAYS set a SL!! Good luck fellow traders
The opening of your post, Tom, reminded me of my own recent near miss:
While looking after my 2 1/2 year old about 2-weeks ago, I left her playing on a piano app on my iPad…I came back to her tapping away on MT4.
Note to all: switch off Multitasking Gestures. She used 4 fingers to swipe a direction which jumped to the next open app (which happened to be MT4). Thankfully she didn’t open a trade but it could have been catastrophic as the iPad was then put away so if a trade had been opened, it would have no stop-loss & it would have been open for absolutely ages.
Major mistake narrowly avoided!
Try using more than one economic calendar, forexfactory is good but check dailyfx or something aswell because sometimes one calendar forgets to put an event in.
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cool, yeah sometimes dailyfx may have an event as low importance but forex factory might have it as medium or high or one would not even mention an event sometimes something important as a central banker speaking. so i found it was good to use a couple of calendars.
The worst thing I have ever done is to take the advice of an advisory broker, that was 15 years ago and the lesson cost a certain amount.
The best thing I have ever done is to learn from that and to become my own advisory broker.
Wow that IS scary! 
- and so easily happens!
I guess nowadays most brokers have a New Trade template that allows settings for an automatic LS and SL every time a new trade is entered? I always have both a limit and a so-called “airbag” stop-loss that set up automatically with every trade and can be fine-tuned afterwards. Once I had a system failure immediately after entering a trade and I would have been living a very nervous half hour before it returned without the comfort of that automatic stop-loss there…
I thought trading is easy and started without any practice. I traded without any trading strategy or money management. Now I’ve learnt and its a different story.
The worst thing which I ever did in forex business, I used huge lot size in single transaction in order to gain huge profit in short time. It took away whole of my account and it was not only my capital which I have invested but my profit has been drained with the losses so it has taken away my profit and my capital in few hours only. It made me realize the importance of good money management and risk management in trading no matter how confident I am.
I have made a lot of mistakes. But there are few I learned a lot from them. For example I was trying to scalp DAX with small account without SL and a politician said something in that moment. I was long and technically it was OK, but the price started to fall suddenly. I learned to avoid excessive exposure then.
Other one was when I tried to open a counter position in the morning to sell the DAX on recent black monday to hedge my long position. I did it on the way to my workspace on my phone and forget to check position size. I opened 5x bigger position because I opened a position there on my Demo account which had much higher balance than my real account. The volatility was ultra high that time and it took few minutes to me to realize my mistake and to close it manually. 20% of account up and down in 5 minutes. I learned to check trade size first I open a trade especially when I change my device.
There are lot of others too, maybe every mistake a trader can do I have already done.