What can happen if you hit someone but no report is made?

I'm not sure what you mean by a "report."  Do you mean a police report?  Police reports have little to do with liability claims.  Liability claims can be made a month, a year, sometimes longer after a collision.  The longer someone waits to make a claim, the more it looks like the claim is minor or insignificant.  If you hit someone, you are liable for their damages.  You can still be liable even if they wait to file a report or make a claim.