Here is a reference to the inacuracies I mentioned:
Are There Inaccurate Carbon Dates?
Yes. There are three kinds.The first kind are datings of things that should't be carbon dated. For example, polar bears that eat seals aren't getting their carbon from an atmospheric source.
The second kind are datings on contaminated samples, or on samples which are a mixture. Old samples contain much less C14, so the measured date of older samples is strongly affected by even small amounts of contamination.
The third kind are dates which were measured before the 1970's. In the 70's:
much better measurement equipment was introduced.
the tree-ring calibration eliminated the assumption about the Sun being constant.
procedures for avoiding and recognizing contamination were established.
In short, all carbon datings published in the 1950's and 1960's are suspect.
This quote came from: http://www.cs.colorado.edu/~lindsay/creation/carbon.html#inaccurate
And here is something about the difficulty in dating Kennewick dude:
Date matches original assessmentThe Interior Department last fall sent bone samples from the skeleton to three laboratories for radiocarbon testing, a process that dates organic material by determining how much of its radioactive carbon has decayed. The test of a foot bone - in which the department is most confident - gave researchers a "raw" radiocarbon age of 8,410 BP, or "before present," give or take 40 years.
A finger bone taken by anthropologist Jim Chatters shortly after the skeleton was found four years ago also produced a raw age of 8,410 years, plus or minus 60 years.
When the raw age is adjusted for changes in the amount of carbon in the atmosphere over the years, the age comes to between 9,320 and 9,510 years.
"Their raw date was very close to ours - eerily close," McManamon said.
Chatters said the recent dates give him a good measure of vindication. Federal researchers had doubted much of the work he did on the skeleton.
"This is fabulous," he said. "You can't ask for more. That's excellent. I'm quite pleased."
Another foot-bone sample taken by Interior scientists had a raw age of 8,130 years, with a 40-year margin of error. Samples from the front of a shin bone produced raw ages of 6,940 and 5,750 years, the latter with a 100-year margin of error.
Researchers told Interior officials that the tibia samples were more exposed to the younger carbon of surrounding sediments, producing the younger ages. The 1996 test was from a bone found among intact sediments in the skull, which might have protected it from contamination, researchers said.
That came from: http://seattletimes.nwsource.com/news/local/html98/kenn_20000113.html
ps. precisely, it's 5730 years (the half life)