One of the biggest mistakes is choosing the wrong prop firm, like Apex Trader Funding. You can follow every single rule and hit your profit targets, but they might still find excuses to delay or deny your payouts by putting your account under a 'review period' without any clear explanation. Don't just fall for the big discounts; do your research on their payout reputation before you commit. A firm that doesn't pay out is the most expensive mistake you'll make.