I suggest the following as a preliminary list of reasons:
- No references at all
- Quality of the reference is "relatively poor", i.e. is no better than the qualify of the reference which contains the notable claim cited in the question: e.g. using published newspaper article as a reference is OK if the question was about a mere rumour, but is not OK (is inadequate) if the claim that's in doubt was carried in a different newspaper: the reference in an answer should be (relatively) higher quality than the reference (if any) in the question.
- Quality of the reference is "absolutely poor", e.g. it's a link to your own blog, or a repeat of the reference in the question
- Author's conclusion is stronger than the reference supports (your conclusion/summary should not be more emphatic/conclusive that supported by the referenced evidence)
- Original research (experiments you conducted at home)
- Unjustified arithmetic (pulling numbers out of a hat)
- Not answering the question (being off-topic)
- Repeating an existing answer
- No references at all
- Quality of the reference is "relatively poor", i.e. is no better than the qualify of the reference which contains the notable claim cited in the question: e.g. using published newspaper article as a reference is OK if the question was about a mere rumour, but is not OK (is inadequate) if the claim that's in doubt was carried in a different newspaper: the reference in an answer should be (relatively) higher quality than the reference (if any) in the question.
- Quality of the reference is "absolutely poor", e.g. it's a link to your own blog, or a repeat of the reference in the question
- Author's conclusion is stronger than the reference supports (your conclusion/summary should not be more emphatic/conclusive that supported by the referenced evidence)
- Original research (experiments you conducted at home)
- Unjustified arithmetic (pulling numbers out of a hat)
- Not answering the question (being off-topic)
- Repeating an existing answer