In the computer field there's a saying -- "Garbage in, garbage out" -- and it applies in photography, too. If the camera isn't set right, it won't take a good picture.
Slip-ups can happen to anyone. One of my most embarrassing goofs happened on a news assignment.
I'd gone with a mountain rescue team to one of our national parks, where a fisherman had fallen over the edge of a cliff but was still hanging onto the ledge that had broken his fall. While the rescue team lowered itself over the edge, I chose a vantage point across the canyon.
I arrived at my shooting position out of breath from the climb, and none too soon. As the action unfolded, I quickly loaded and started to click away. After shooting the roll, recording the actual rescue of the sportsman as he was lowered from the ledge, I started to rewind my 35mm camera to change rolls.
A couple of turns on the rewind knob and the film rolled free. I had a sudden, sick feeling in my stomach. Sure enough, the film hadn't gone through the camera: I'd been shooting air. The film-counter had worked and the shutter had clicked, but in my excitement I hadn't noticed the absence of the customary drag of the film being pulled as you cock the shutter. The film had slipped out of the sprocket holes.
That's when I learned to snug up the rewind knob after loading, and then watch the knob rotate while advancing the film for a couple of blank exposures.
Other common errors are just as easy to correct. For example: Have you ever set the wrong ASA for the film you're using, or forgotton to change it when changing rolls? How about the wrong color film -- like type B (indoor) for outside daylight shooting?
Then there are some mistakes almost too embarrasing to mention: leaving the lens cap on, cutting off heads or feet or using the wrong lensshade on a wide-angle lens so that all your pictures look like they were taken through the porthole of a ship. But there are ways to avoid these slip-ups.
When changing film, always check the ASA number of the film you're using against your dial setting and set it correctly.
Be sure to set the correct sync-speed on your shutter dial when shooting flash. If you don't you may get only part of an image. Sync speeds vary from 1/30th to 1/125th depending on the camera, and you will get a complete image at that shutter setting or slower -- but not faster. If you don't know the sync speed of your camera, use the slowest setting until you can check it out.
If you are changing from outdoor to indoor color film, the best way to remember what you loaded is to tape the label from the box to the camera back. If you use indoor (type B) color film outdoors, everything will turn blue. And if you shot outdoor film indoors with tungsten (bulb) light, your pictures will have a reddish cast as though they were taken with a red filter over the lens. (Of course sometimes you may want these effects on purpose; but the wrong film by mistake can ruin your pictures.)
Leaving on the lens cap can be cured by putting on a lens shade and taking off the cap before you start shooting. (Of course you won't have this problem with a through-the-lens SLR camera.) Remember that every lens has its own size shade: They're not interchangeable.
True, you can use the shorter focal-length lens shades on the teles, but they won't be as effective. And if you go the other way and use the normal-lens shade or tele-lens shade with the wide-angles, you'll be putting into the picture. The bad part about this vignetting is that you won't see the porthole image when you shoot because you're looking through the lens wide open -- but it'll show up later on the stopped-down picture.
Heads, feet and other parts of a picture are usually cut off because the eye is not close enough to the viewfinder. If you can't get your eyes close enough because you wear glasses, and as a result don't see all of the image, buy a diopter attachment so that you can see without glasses. Another solution if you're in doubt about your field-of-view is to turn the canera vertically and give your subject more head- and foot-room.
Despite all advancements, even the best camera won't correct for human error.