Hakeemul Ummah Hazrat Moulana Ashraf Ali Thaanwi Sahib رحمه الله has said that if someone performs five rakaats instead of four rakaats for Zuhr, then even his four rakaats will not be accepted. This person may claim that he has not committed any sin by adding an additional rakaat which is nothing but Salaah. On the contrary, he can say that he has performed better by adding a rakaat.
However, his Salaah is not accepted because he has violated the law. While everyone understands this fact, they do not apply the same logic to their other acts of bid’ah. They claim that their acts of bid’ah are deeds of virtue.
(SOURCE: MALFOOZAAT OF HAZRAT MOULANA ASHRAF ALI THAANWI SAHIB رحمه الله, PART 6)