On what grounds does the USPTO usually refuse specimens?

Common grounds for the USPTO to issue a specimen refusal include:

• The specimen being illegible.
• The specimen appearing to be digitally created or altered and not a real photo or screenshot.
• The mark in the specimen not matching the originally filed mark.
• The specimen not proving the mark's use in commerce in the applied-for class, which happens when the mark in the specimen labels a product that doesn't belong under the class where the applicant is seeking registration.
• The website specimen for goods not including any means of ordering the goods (relevant for goods, not services).
• The specimen comprising of advertising material (relevant for goods, not services).
• The website specimen for goods showing the trademark labelling the website and not the goods themselves.
• The specimen not showing the applied goods at all, which happens when the photo is too close up, or the specimen shows closed packaging without any indication of what the product is.
• The specimen showing the mark used in a way that is merely decorative (or "ornamental") rather than as a trademark - a sign capable of identifying the source of goods/services. For example, a slogan put on a shirt would probably receive an ornamental refusal.