What happens if you fail pre emoyment- synthetic

I honestly have not seen any post where one failed submitting synthetic...I'm sure it happens... When it does will call come from employer or MRO?
Just comes back as diluted? What should I say to mro or employer if it happens? I just say I drank alot of water before test???Allowed to take second test? Will I definately be supervised? Labcorp was lab.
Just need some directions in case it happens don't know what to do or say.
Submitted agent-x four business days ago not a word from labcop or employer. Just very nervous don't know when I can relax