As friends of the bereaved, we usually offer our support and concern by giving them our sympathy greetings. But sometimes, when we haven't experienced grieving ourselves, we tend to give hasty words offering our sympathy, only to find out later on that they're not appropriate.
For our friends who are still mourning for the death of their loved ones, try not to say these to them:
1) "Move on". This is perhaps the most inconsiderate thing to say to someone grieving. Only the bereaved can only tell when he's ready and when he can move on. Grieving must be done on its own time and it may take quite a while (or a long time) for someone to get over, how much more to move on to a loved one's demise.
2) "Don't cry." Especially during the early days of mourning, one shouldn't say this to someone, even simply out of one's concern for their friends. Crying is as natural as smiling and laughing, or anything. You shouldn't deprive someone of expressing the way they feel, Crying is a good therapy.
3) "I know how you feel" Nobody TRULY knows how the bereaved does except himself, though you experienced grieving at some point in your life.
4) "You should not feel like that." Nobody has the right to tell someone how one should feel at a given time.
5) "Time will heal"
And there are other typical statements we casually say that we've no idea can be inappropriate and uncalled for.
For more stuff on what you should refrain from saying to someone mourning, I released a new ebook Healing After Death.