Mouse is one of the most common production cider faults, named after a mousy smell which taints the cider caused by a family of alkyl tetrahydropyridines which are generated by infection from certain strains of lactic acid bacteria.
It can be prevented through thorough cleanliness and possibly by proper addition of sulphur dioxide to the juice before fermentation and during storage. Even then it occasionally rears its head unexpectedly.
It is worth pointing out that the human perception of 'mouse' is very idiosyncratic and varies wildly from person to person. It may be genetically determined. One key factor is the pH of the oral cavity. People with high pH saliva convert the pyridines more quickly into the volatile free-base form and so are much more sensitive than people with low pH saliva where the 'mouse' stays in the non-volatile salt form and hence does not interact so much with their olfactory receptors.
There is a little trick which increases sensitivity to "mouse". Mix a bit of baking soda in a glass of water and rinse your mouth with it before tasting. This temporarily increases oral pH, allowing the fault to be better detected. But the reaction doesn't have to happen in the mouth. One can add baking soda to a cider sample to increase the pH, and the mousiness will come out in the nose. This has the distinct advantage that you don't have the residual taste in your mouth (and it's more than a bit of a disgusting taste).