Dendrochronology is suspect because 2 or more rings can grow per year

Claim

Two or more growth periods frequently occur during a year, so dates derived from tree rings (dendrochronology) are suspect.

There are two systematic errors in the measurement of time by dendrochronology: Missing rings and additional rings. Additional rings go in the right direction for creationism, making the ages seem too large, but the effect is not only far too small and easily spotted, but also loses in effect size against the missing rings, which can make the trees appear about 5% younger.
"In certain species of conifers, especially those at lower elevations or in southern latitudes, one season's growth increment may be composed of two or more flushes of growth, each of which may strongly resemble an annual ring. Such multiple growth rings are extremely rare in bristlecone pines, however, and they are especially infrequent at the elevation and latitude (37° 20' N) of the sites being studied. In the growth-ring analyses of approximately one thousand trees in the White Mountains, we have, in fact, found no more than three or four occurrences of even incipient multiple growth layers." (See Ferguson, C. W. (1968). "Bristlecone Pine: Science and Esthetics." Science 159(3817): 839-846.)
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Quibbling (a few additional rings can't make a tree appear older by the factors needed by YECs)
Suppressed Evidence (missing rings are ignored)

Mark Isaak's page for this claim [1]
Matson, Dave E., 1994. How Good Are Those Young-Earth Arguments? A Close Look at Dr. Hovind's List of Young-Earth Arguments and Other Claims. [2]