I noticed that using a field number of SIZE_MAX or larger makes --debug give an invalid diagnostic:
$ :|_POSIX2_VERSION=199209 src/sort --debug +$(echo 2^64-1|bc).4 -1.2 src/sort: using simple byte comparison src/sort: obsolescent key `+0 -2' used; consider `-k 1,2' instead src/sort: leading blanks are significant in key 1; consider also specifying `b'
