For independent trades, the chance of exactly n losses in a row starting at a given trade is q^n where q is the loss rate. At q = 0.6: five in a row is 7.8%, eight is 1.7%, ten is 0.6%.
The trap is reading those as the probability of ever seeing such a run. Across 300 trades there are roughly 300 starting positions, and the chance of at least one run of eight is far higher than 1.7% - closer to 40%. Rare per attempt, routine per career. This is the same counting error that makes sharpe-inflation possible.
One caution on independence. Trades taken in the same regime, in correlated instruments, or while tilted are not independent, and real streaks run longer than the formula predicts. Treat the calculation as a floor on how bad a normal run can get, not a ceiling. See gamblers-fallacy for the opposite error - believing a long streak makes the next outcome more likely to reverse.
Related: consecutive-losses, loss-rate, gamblers-fallacy, risk-of-drawdown