Overstaying a Thai visa carries real, escalating consequences, and understanding the basic shape of the system is worth knowing even if you never intend to test it.
The daily fine
Thailand charges a daily fine for overstaying, up to a capped maximum — paid at the airport on departure or at an immigration office, and generally handled without major incident for shorter, unintentional overstays.
Longer overstays
Extended overstays carry more serious consequences, including possible detention, formal deportation, and re-entry bans of varying length depending on how long the overstay ran — a genuinely different category of problem from a short accidental overstay.
Extending a visa properly instead
Thai immigration offices, including one on Phuket, process visa extensions for many visa categories — the correct route if you want or need more time, rather than simply staying past your permitted date and paying the fine on the way out.
Border runs
Some travellers historically extended their stay by briefly exiting and re-entering Thailand — rules and enforcement around this practice have tightened over time and vary by visa type, so treat it as something to research current rules on directly rather than assume still works the way it may have in the past.
Bottom line
Track your permitted stay date carefully from your entry stamp, and if you want to stay longer, visit an immigration office to extend properly rather than risk an overstay — this is one area where getting it wrong has real, lasting consequences.