The NKJV in Mark 3:29 has "but is subject to eternal condemnation" and the NASB has is "guilty of an eternal sin". Are the meanings different? They seem to mean something different. Isn't the NASB more accurate to the original then the NKJV?
Both the NASB and the NKJV are accurate translations. However, they use different rules regarding the source text when a variation is found. The NKJV tends to follow the Byzantine texts, which we have more copies of (which leads to it being called the "Majority Text"). The NASB tends to follow United Bible Society's text which tends to favor older manuscripts. In this particular verse there appears to be an early split with the last word in Mark 3:29. The Byzantine and three other old manuscripts have kriseos (judgment). Eight other manuscripts, some of equal age or older have hamartematos (sin, an act of disobedience).
The two are different in meaning. The NKJV, if it is correct, says that the person is eternally condemned because they are never forgiven. It is the same was found in Matthew 12:32 when Jesus said it would not be forgiven in the age to come (that is, in eternal life). The NASB, if it is correct, addresses why there is no forgiveness. People guilty of this sin never repent and give this sin up, even in eternity. As The Complete Biblical Library notes: "Lastly, there is no possibility of committing this sin accidentally. Nor is any who has committed this sin likely to feel grief for it. Its heinousness lies in its premeditated unwillingness to recognize God as the source of righteousness."
The end result it the same, whichever variation is the correct one. One addresses the result, the other addresses why. It is unfortunate that the variation exists, but neither variation really changes the meaning of what Jesus said.
Another interesting note is found in the NET Bible: "This passage has troubled many people, who have wondered whether or not they have committed this eternal sin. Three things must be kept in mind: (1) the nature of the sin is to ascribe what is the obvious work of the Holy Spirit (e.g., releasing people from Satan’s power) to Satan himself; (2) it is not simply a momentary doubt or sinful attitude, but is indeed a settled condition which opposes the Spirit’s work, as typified by the religious leaders who opposed Jesus; and (3) a person who is concerned about it has probably never committed this sin, for those who commit it here (i.e., the religious leaders) are not in the least concerned about Jesus’ warning. On this last point see W. W. Wessel, “Mark,” EBC 8:645-46."