What are the consequences of visa validity mistakes for entering Thailand?

Expensive mistakes.

Man and two children with German passport and lady with Serbian passport.

Tourists bough a nice holiday trip to Thailand, the German tour office tells the lady needs a tourist visa based on her passport. She goes to Embassy and applies long before the holiday and gets the tourist visa. The date is there and off they go to Thailand. On arrival husband and children no problem, the lady get stopped by Immigration because her visa is invalid. It is a few days later than the enter before date. After a few phone calls the tour office admit their mistake and the lady is back to Germany, going to apply for a new Visa and joining after a few days the rest of the family in Thailand. Days of holiday changed into flying up and down and getting new Visa.

As I see it: Mistake from tour office by advising go early for visa, mistake from Embassy for not checking the ticket date, mistake from airline company by not checking the visa by check in, mistake from the person by not informing herself enough about the visa rules.

Very expensive lesson learned.

Always check the dates on every visa you apply for and get, always check the dates of every stamp they put in your passport. Or have a credit card in case mistakes are made.

Edit: This is a real story and happened at Suvarnabhumi Airport yesterday 19 Nov 2017.