Combining -v and -A/-B breaks

[Terje Eggestad]

It appears -v with -A or -B is broken on 2.5.1 (on RHEL 5.9) as well as
2.14 (fedora 18)
When combining -v & -B   grep seem to become cat
(I could accept that it's an illegal combination, but then grep should give
an error message)

$ cal | grep 18
16 17 18 19 20 21 22
$ cal | grep -B 1 18
 9 10 11 12 13 14 15
16 17 18 19 20 21 22
$ cal | grep -v  18
      June 2013
Su Mo Tu We Th Fr Sa
                   1
 2  3  4  5  6  7  8
 9 10 11 12 13 14 15
23 24 25 26 27 28 29
30
$ cal | grep -v -B 1 18
      June 2013
Su Mo Tu We Th Fr Sa
                   1
 2  3  4  5  6  7  8
 9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30