Always check any unexpected faults on your print first against the corresponding image on the negative. Assuming that the negative itself is free of blemishes, the most common faults in printing and their causes are:
White specks and hair shapes, due to debris temporarily lying on the paper surface, the negative or any carrier glass. If marks are very unsharp the dirt may be on a condenser or diffuser surface between the negative and lamp. Uneven patches of density, which may be quite large, caused by not submerging the print quickly and evenly in the developer. Perhaps development time was impracticably short. Small whitish patches with distinct edges, caused by water or wet finger marks on the emulsion before processing.
A purple patch, or purple all over. Print insufficiently fixed, and therefore reacting to white light. Fine black lines, often short and in parallel groups, caused by physical abrasions of the (dry) emulsion, perhaps from dropping the paper on the floor or slipping it roughly under the masking frame masks.
One or two short, thick black marks close to a print edge. Caused by over-energetic gripping by print tongs in the developer. Only part of the image being sharp. The negative is bowed, or at an angle to the paper. Part of the picture showing an offset, double image. Probably caused by the masking frame, lens or negative being jogged between your main and printing-in exposures. Grey, muddy image, with smudged shadow detail and sometimes veiled highlights. Caused by grease, dust, or temporary condensation on the lens. A slight fog-like dark band close to the print’s white border, due to light spread from the rebate of the negative. Contact prints unsharp. Insufficient pressure between cover glass and paper.
Collapsed blisters in the emulsion surface, where it has separated from the base. The print was not fully covered by a film of water when passed through a roller RC heat dryer. White areas, including borders, are veiled with grey. Extreme over-development, or fogging by your darkroom safe-light
