Alrighty, this one had me stumped for a while:

$ cat Makefile
DIRB =dirB
DIRC = dirC
DIRD=dirD# immediate comment
DIRE= dirE# immediate comment
DIRG=dirG # (dirF has a trailing space too)
FILE = a  # more spaces here

make : $(DIRA)/$(FILE) $(DIRB)/$(FILE) $(DIRC)/$(FILE) $(DIRD)/$(FILE) $(DIRE)/$(FILE) $(DIRF)/$(FILE) $(DIRG)/$(FILE)
        echo "done"
% :
        echo "looking for $@"
$ make
looking for dirA/a
looking for dirB/a
looking for dirC/a
looking for dirD/a
looking for dirE/a
looking for dirF
looking for /a
looking for dirG

Apparently bonus whitespace (normally before a comment or after an equals sign) in variable definitions is stripped from the front of a definition, but not the back. You have been warned ;).