1)   1) One should insist that Amen be said accurately. It follows from this that one should not say a “snatched” Amen, which refers to an Amen which was started before the one making the blessing had finished his blessing. In the same way, he should not say the Amen in a way that it sounds as if the letter “Alef” is vowelized with a “Sheva”. One should also not pronounce a “chopped” Amen when one does not pronounce the “nun” at the end of the word. (Shulchan Aruch and Mishne Berurah 124)

2)   2) One should not say an “orphaned” Amen, when he does not actually hear the blessing himself but hears others responding Amen. According to the view of the Shulchan Aruch this law applies only when responding to a blessing which he would like to use to fulfil his obligation, and even if he knows to which blessing he is responding. According to the view of the Rema this law applies also to other blessings, but only when he does not know to which blessing he is responding (in which case he cannot fulfil his obligation either). (It follows that if he knows which blessing has been said, and he has no intention to fulfil his obligation with it, he may respond Amen according to everyone). (ibid)