Various creationist books mention the 1801 eruption of the Hualalai volcano in Hawaii. They claim that scientists dated the 200 year old lava, and got the utterly bogus age of 22 million years. Some books mention other bogus ages: 140 million years, or 2.96 billion years.
The claims are false. The scientists in question reported the lava's measured age as approximately zero. (Potassium's half-life is 1.3 billion years, so the imprecision - the "slop" -in their potassium/argon dating was much larger than 200 years.)
The scientists did date something as being million of years old, but it wasn't lava. Notice the title of the actual scientific report:
Radiogenic Helium and Argon in Ultramafic Inclusions from Hawaii, J.G. Funkhouser and J.J. Naughton, Journal of Geophysical Research 73:14 pp. 4601-4607 (15 July 1968)
The important word here is "Inclusions". The scientists were not out to date the lava: they were dating some chunks of olivine that were stuck in the lava like raisins. This lava was not hot enough to melt olivine, so the chunks were carried along when the eruption brought lava up from the depths. Of course the olivine inclusions dated as being old. They are old.
The article reports that the theory was correct. K/Ar dating should not be used on xenoliths. But, the article clearly states (on page 4603) that the surrounding lava was dated correctly. This article casts no doubts whatsoever on the dating of lava.
For further reading:
Faure, Gunter. Principles of Isotope Geology, 2nd edition, 1986. Wiley, New York
The Age of the Earth, Dalrymple
Last modified: 7 July 2012