Like most people I've made silly mistakes, sometimes it has gone in my favour and other times not, but overall what I have lost has been a pittance compared to my overall profit.
Here are some things I wish someone had told me before I started:
1. Take your time. Never do offers if you are in a rush or tired, especially when first starting out.
2. Avoid in play offers until you are confident. You will need to be quicker at placing in play bets as the odds can change quickly - eventually you will be able to do this no problem, but don't put yourself under that pressure until you are ready
3. Others might not agree with this one, but I would avoid horses at first and concentrate on football. There is more that can go wrong when betting on horses (your horse could be a non runner for example), these things are usually correctable but again, get confident with the general process first
4. Always check there is enough liquidity in the exchange to lay your bet BEFORE you place your back bet. This would have saved me a lot of stress when I started
5. Familiarise yourself with the process of how to correct it if a lay bet is only partially matched. If there is plenty of liquidity in the market and its a pre-match bet then you shouldn't have a problem with this, but when you branch out into in play offers this can happen. Its easily corrected and the have a calculator for it on PA, but having some advanced knowledge can prevent a mass panic (when more mistakes could occur)
6. Finally, when you branch out into casino offers ALWAYS check the spin amount before you start spinning!
I can honestly say I haven't had any major disasters and I don't know anyone who has. Like with anything, you will probably make the odd mistake but what you make in the long run will far, far outweigh anything you lose.