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A general description is here.
R CMD INSTALL no longer gives a spurious warning about old R versions ignoring multiple dependencies, if the conditions are known to be satisfied.
Invalid use of sprintf() such as sprintf("%S%") now give an error instead of a segmentation fault, as do very unusual cases such as sprintf("%s", tryCatch(stop(), error=identity)). (It was always documented that misuse could crash R in platform-dependent ways.)
The digamma(), trigamma() and psigamma() functions could be inaccurate for values of x around 1e-15 due to cancellation. (PR#13714).
median.default() was altered in 2.8.1 to use sum() rather than mean(), although it was still documented to use mean(). This caused problems for POSIXt objects, for which mean() but not sum() makes sense, so the change has been reverted.
Assigning an extra 0-length column to a data frame by DF$foo <- value gave a corrupt data frame rather than failing. (PR#13724) This also happened for DF[["foo"]] <- value.