Sin unto death

What is the "sin unto death" that we should not pray for someone 1 John 5:16? Has this topic been discussed before? If so how do I find it?

Commentators typically suggest this is either the blasphemy of the Holy Spirit or it is like a sin that causes God to kill you like Ananias and Sapphira in the book of Acts.

Since both James and John speak of two kinds of sin, and both speak of a kind of sin that produce death, I think it is safe to compare the two verses. James says: sin that is perfected produce death. Now you have to find out what is a perfected sin. I think it is also helpful to find out what kind of death is John talking about, physical death or spiritual death. I believe he means the latter.

The only sin Jesus pointed out as unforgiveable is blaspheming the Holy Spirit when the attributed him working by a demonic spirit.

Mat 12:31 ¶ Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy [against] the [Holy] Ghost shall not be forgiven unto men.
12:32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the [world] to come.

Mat 12:24 But when the Pharisees heard [it], they said, This [fellow] doth not cast out devils, but by Beelzebub the prince of the devils.

If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death ; I do not say that he should make request for this. All unrighteousness is sin, and there is a sin not leading to death.

Because this is a believer's sin, it cannot be spiritual death (in a final sense) or else we are all to be pitied. In addition, John says some sin does not lead to death.

As AReadyMind pointed out, it is when God passes a final judgment on an individual. Ananias and Sapphria is one of the first examples people think of. The believers in Corith who fell asleep because they profaned the Lord's Table would be another. When Moses was sent to the mountain top to die is a more ancient example.

The common thread in all of these is that the person(s) failed to maintain God's holiness in their public witness. The severity of the sin committed and the time God allowed the sin to be commited varies quite significantly, as you can see.

Matthew