The conclusion of a standard form categorical syllogism is affirmative, but at least one of the premises is negative.
This is fallacious by definition, since it does not respect syllogistic structure. All validating forms of categorical syllogism which have one negative premise also have a negative conclusion.
All mice are animals, and some animals are not dangerous, therefore some mice are dangerous.
No honest people steal, and all honest people pay taxes, so some people who steal pay pay taxes.
Assume that the premises are true. Find an example which allows the premises to be true but which clearly contradicts the conclusion.