Depressing List Of Things That Can Go Wrong
( Film processing )

My film is completely black.
Indicates gross overexposure. If the film is all black including the rebates then the film has been exposed to light and completely fogged.

My film is completely clear.
Indicates gross underexposure. If there really is nothing there at all apart from edge numbers, you probably didn't expose the film at all , either through incorrect loading where the film hasn't wound on or by developing an unused film by accident. If the film is completely clear including the rebate and the leader then you have fixed the film before developing .

Negatives are very thin ( faint ).
Assuming correct exposure in the first place. Underdevelopment. Developer too cold, too short development time, developer to dilute or insufficient agitation.

Negatives are very dense ( dark ).
Assuming correct exposure in the first place. Overdevelopment. Developer too hot, too long development time, developer too concentrated or over agitation.

Film is cloudy or milky.
Insufficient fixing. Too short fixing time, fixer exhausted or fixer too diluted. Return to fixer solution or if necessary re-fix in a fresh solution.

Uneven development.
1) Film is lighter or underdeveloped along one side . Not enough developer to cover the film in the tank.
2) Negatives have denser areas mirroring the shape of the sprocket holes. Surge marks caused by vigorous agitation causing developer to surge through sprocket holes.
3) Undeveloped patches or patches of film which have been unaffected by any processing solutions. Film incorrectly loaded onto spiral allowing film surfaces to come in contact with each other inhibiting or preventing chemicals from acting on surfaces.

Surface damage.
1) Dust embedded in surface. Dirty water or ,more likely, dust settling on film while it is hanging up to dry.
2) Scuff marks or scratches. Caused by rough handling. Although physically quite strong film is easily scratched, particularly the emulsion side when wet.
3) Straight scratches extending the length of the film. Tramlines. Caused by dirt or grit either on the felt light trap on the film cassette or on the film plate in the camera. Tramlines can also occur if you squeege or run wet film through your fingers to remove excess water before drying. Wet emulsion is very delicate and even a small piece of dirt or grit will leave a scratch the length of your film.
4) Roughly circular marks on film surface. Drying marks. Droplets of water form on the film as it is drying. As they dry out they leave residue on the film surface. If marks are on the non-emulsion (shiny) side it may be possible to very gently polish them off, though I am not actually recommending this.
5) Clear crescent shaped marks. Caused by kinking or buckling the film during loading. Forcing the film onto the spiral or cross threading the film in the grooves.