On the early cars with no warning light, its really easy to forget to latch it all the way - I have been guilty of this myself. Its not a huge hazard as the primary latch typically engages simply by the weight of the hood which will keep it from flying up.
Back in the day, I had a Ford panel van and I forgot to latch the small hood and it came up, broke the metal hinges, cracked the windshield, and flew off over the top of the van as soon as I got up to about 50 mph. Luckily, no one was behind me.
