While searching through the internet for common mistakes photographers do, I encountered a very good website that talks about the very basic mistakes. You'll probably laugh at these, but these are mistakes that most people make, even if they have taken photographs before. Let's not even talk about rule of thirds or handling difficult lighting situations. Most of these mistakes relate to basic composition and framing and lighting. This website provides scanned copies of real printed photographs so you can see that it is not much different. Most of these are very much applicable to digital photography as well except for the 3rd and the last, which are unique to the use of films. As you probably already know, the only advantage of digital photography is that you can probably filter out these mistakes immediately after taking the shot, and thus, have the opportunity to take another shot.
Here's a summary of the common mistakes presented by the author. I suggest you visit the website. It's a good opportunity also to get a glimpse of photography in the early years.
1.) The photographer's own shadow is in the picture.
2.) Distance from subject: Too close, too far away
3.) Forgot to advance the film: The Double Exposure
4.) Blocked the lens with a finger or a thumb or something
5.) Somebody moved, either the subject or the photographer
6.) Not framed right, Missing heads, feet, people
7.) Improper lighting (Overexposed, Underexposed)
8.) Fogged negative from miss-loaded film or a damaged camera.