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A general description is here.
col(x, as.factor=TRUE) and row(x, as.factor=TRUE) would fail if x had no column (row) names. It was documented incorrectly, and now returns a matrix factor.
setRepositories() failed if "pkgType" was set to "mac.binary.leopard".
The pdf() device can now handle the case where an image uses a very large number of colours with different levels of semi-transparency (alpha-values). It used to segfault. (Also PR#13264)
If all the packages had dependency lists of equal lengths, occasionally install.packages() would compute an incorrect sort order.