Overstaying a US tourist visa is considered an immigration violation and can lead to a 3-year or 10-year ban from re-entering the US, your visa being cancelled, being deported, and making it harder to get visas later. The specific consequences depend on the length of the overstay: staying more than 180 days but less than a year can lead to a 3-year ban, while overstaying more than a year gives you a 10-year ban.