Dear Sir / Ma'm, if there are any discrepancies in the NEET UG 2019 question, the NTA will release a notice regarding it and also the amount of grace marks which will be awarded (if any) after the display of the OMR sheet which will be tentatively around the 4th week of May, 2019, after which you can challenge the OMR sheet if you feel there is a problem. However, for any official notice regarding grace marks or discrepancies in question, NTA is bound to release an official notice.