"grep -F -w" fails to match with other non-matching lines present

[Göran Uddeborg]

If I create two files

pat with two lines:
    Date
    DateTime

and text with three lines:
    Date
    DateDC
    DateTime

Now, if I do "grep -w -f pat text" I get two lines

    Date
    DateTime

but if I do "grep -F -w -f pat text" I only get the first line.  If I
remove the non-matching DateDC line from text, it works again.

Some version info:

morgantown> uname -a
Linux morgantown 2.4.21-15.0.3.ELsmp #1 SMP Tue Jun 29 17:46:55 EDT 2004 x86_64 
x86_64 x86_64 GNU/Linux
morgantown> grep --version
grep (GNU grep) 2.5.1