Vignetting.

[Mike Kennedy]

If you can't see it through the viewfinder can it occur on a print?
Example:My beater/storm camera is a Nikkromat FTn with 50mm Nikkor lens.Found a metal lens hood (for 135mm lens) that keeps the snow/water off my front element.I can't see any vignetting but could it happen as the view I get is about 97% of the actual image.

 

[David Brown]

Yes. I am assuming it's on the negative, too. If it's only on the print, there's another problem.

There is a reason why there are different hoods for different focal lengths. It's not just marketing. It is likely that a hood for a 135 is too long for your 50.

97% is probably optimistic

 

[John Koehrer]

Yes it will vignette. It may be visible if you stop the lens down when viewing.

 

[Larry.Manuel]

I made a lens hood for my Art Deco Rolleicord, and it vignetted the film at first. A 35mm film canister fit nicely over the taking lens. Here's how I determined the correct amount to cut it back: With the shutter on "T", and looking through the lens from the film side, it was easy to see how the hood was vignetting the corners. My viewpoint was through the lens, from the extreme corners of the film plane. Lens was wide open - a mighty f/4.5.

 

[asp.artist]

Someone posted a link to a site with info on how to make your own lens hood. lenshood.co.uk. Looks interesting.
Anne