d = as.data.table(lapply(1:10, c))
options(datatable.verbose=TRUE)
forderv(d, by=c("V1","V2"))
#forder.c received 1 rows and 10 columns

Those 10 columns are actually irrelevant, because AFAIK we are not even touching other columns than those provided in by.

This is not really that important so lets have this issue as a documentation, that we can refer to it from tests that are affected. Will mark it closed.