Hi
Backtests do not reflect the real world.If a broker increases his spread during periods of high volatility from 3 pips to 15 pips ,back tests will only show the last bid offer based on penultimate price.
If u get a bucket shop increasing spreads 10 times a month and a trader pays 10 pips extra per trade *50 % of the time = 50 pips loss in real trading.This is not shown in back tests of data provided by broker
Other factors include corrupt data,computer breakdowns and internet connection failure
Best to live test on 0.01 lots on real account
OILFXPRO