Just came to know about someone this weekend through my friend, who did Tax class fraud and got caught...

The person was married in his home country, brought his wife on family visa. He changed his tax class from 1 to 3 based on marriage. The wife lived in Germany for 5 months and then left Germany. They got divorced 3 months after she left Germany in their home country.He did not inform ABH and other authorities about the chnage of his marriage status plus that wife is not living in Germany and continue paying taxes on class 3.
After 2 years, Fianzamt found this with the help of ABH and now he has to give back all the money (calculated based on Tax class 1) plus fine of 8000 Euros.

He earn a bad name for himself and for his home country.So it is always better to be fair and tell the authorities correct information's on time.

Right punishment. In tax class 3 a person gets more money so that he/she can maintain the life for her/his partner who is not working. Now if you get the extra money without having to support your partner then you are stealing money from govt. and must be punishable. That is the reason why rules are so important and I am happy that Germany follows these rules strictly.