You can't toss in unregistered shares to the public.
Understood that unregistered PIPE shares can be sold only to “qualified” buyers.
So, if you are selling shares on the shelf and don't have enough, just register more.
It takes time to file and activate an updated shelf registration and this precludes an issuer from exploiting a fleeting moment of positive publicity to raise money on relatively favorable terms. Thus, gfp’s scenario of a mixed financing could make sense in a case where a shelf registration had too few shares to accommodate the entire financing.