When entering Thailand it would be a good idea to make sure you know about your duty as a traveller into Thailand. When entering Thailand the immigration will give you an entry-stamp ** with a certain date until when you can stay in Thailand. This date depends on your visa & nationality. This date gives you the permission to stay in Thailand until that date. But ALWAYS check if this date is correct, because it is your passport and YOU are responsible for a correct entry-stamp. If you think the entry stamp is not correct then STOP and GO BACK to immigration.
Example you do not have a visa and have France nationality. The immigration should give you a visa-free entry-stamp of 30 days. But the immigration officer can make a mistake and gives you 60 days. Happy!! No! Because YOU could get in trouble for that.
How? When you leave Thailand after 55 days and the immigration officer puts your name in the system it will say you are on overstay of 25 days. At 500 Thai baht per day! YOU should know that the entry-stamp was wrong and YOU are responsible for that.
Now lets follow the example, you sit in your hotel and after 1 week you are looking into your passport and realise that you have a wrong entry-stamp. Of course you want to correct that, so you go to the local immigration office. And the immigration officer says "No problem". Do not accept that! Let them correct the entry-stamp. Because it will be another immigration officer at the airport who will see the overstay and all people with something wrong always say : But he said "No problem".
Bottom line : Check your entry-stamp when arriving in Thailand.
And actually check all stamps any immigration officer puts in your passport. If anything wrong let them correct it.
** = A Entry-stamp should have the name of the immigration point, visa-class, Date admitted until and be signed by the immigration officer. This stamp should be in you passport and on your TM-card that should stay in your passport.