I think common mistakes to avoid when using a betting system in online Roulette include overestimating the system's effectiveness, as no system can overcome the house edge. Another mistake is chasing losses by continually increasing bets, which can quickly lead to significant losses during a bad streak. Players often fail to manage their bankroll properly, betting too much of their total funds at once. Relying too heavily on patterns or streaks, such as believing certain numbers are "due," is another pitfall, as each spin is independent. Lastly, ignoring table limits can cause issues, especially with systems like Martingale, where the required bet size may exceed the limit after consecutive losses.