Gst multiple tax rate charged

Respected Sir/Madam,

I have a client in Manufacturing business. They Manufacture component which is used in Parts of Automobile(including Tractor) and Motors.They have charged multiple tax rate to same customer for same material(i.e. 18% and 28%). Wanted to know what would be the consequences as per GST law in following cenario:

1. Charged higher rate and liability adjusted againts the input credit

2. Charged lower rate

3, Wrong HSN code used.

Thank you in advnace.

Vivek solanki