Many years ago I pointed a Canon T90 spot meter at bright noon-lit snow under a blue sky and wondered why it looked so pasty on Kodachrome..........
Many think that you select the spot meter, point it anywhere at the scene and everything will be fine. This is certainly not so. Luck happens though, and you might just get a stroke of it.
Like any other meter, a spot meter will base it's observation on a baseline 18% mid-tone grey. It does not know (or care) if the scene is much brighter or darker than that. If you point it at a big blue sky, a white snowfield, or a black cat, what do you imagine might happen?? Well, none of those are actually 18% grey, are they? I chiefly add +1.3 to a snow scene (+0.3 if overcast).
Correct use of a spot meter is to seek out a tone in the scene being viewed that approximates the ubiquitous 18% grey midtone and meter from that, lock it in then shoot. A spot meter is useful for very strong backlight but will most likely need your over-ruling input. There are a few cameras about that allow multi-spot/averaging (i.e. Olympus OM4). The skill of the photographer will ultimately determine the best result with spot metering.
Get in the habit of reviewing your camera's settings before starting the shoot, but there is no harm in observing the results of your error as a valuable learning experience, remembering also that reversal film will show exposure errors from whichever meter is in use more acutely than neg. stock.
