Common mistakes when using a progressive betting system in online Roulette include chasing losses, where players continue to increase bets in hopes of recovering previous losses, leading to significant financial risk. Overestimating the bankroll is another mistake, as progressive systems like Martingale can quickly escalate bet sizes, exhausting funds or hitting table limits. Players may also ignore the house edge, falsely believing the system guarantees success, when in reality, long losing streaks can still result in significant losses. Lastly, emotional decisions during losing streaks can cause players to deviate from their strategy, worsening outcomes.