Bad tasting mussels and avoiding buying them
Always check that most of the mussels are alive when you buy them.

Check that the shells are not broken, and that they are wet.

At your house, throw away all the dead mussels and process as soon as possible. 

Cook (steam) as fast and as furious as you can for no more than two minutes. At that point, the mussels should have opened and nearly done.

Overcooking can produce a rubbery texture and spoil the taste.