Hi.

whitespace-buffer does not delete its overlays but adds more and more
onto the buffer.

try this to reproduce:
1.:
emacs -Q
C-x C-f test.c (attached, just contains int         foo=1;
                seperated by a space followed by a tab)

2.:
M-x whitespace-buffer
put point on the highlighted overlay and press C-u C-x =
this shows

  character: SPC (040, 32, 0x20, U+0020)
    charset: ascii (ASCII (ISO646 IRV))
 code point: 32
     syntax:           which means: whitespace
   category: a:ASCII   l:Latin
buffer code: 0x20
  file code: 0x20 (encoded by coding system undecided-dos)
    display: by this font (glyph code)
     -outline-Courier New-normal-r-normal-normal-13-97-96-96-c-90-iso8859-1 (0x20)

There are 2 overlays here:
 From 14 to 15
  face                 [whitespace-highlight-face]
 From 14 to 15


There are text properties here:
  fontified            t

repeat 2. to see that now there are 3 overlays
repeat 2. to see that now there are 4 overlays
...

It seems whitespace.el does try to delete some overlay by calling the
function whitespace-unhighlight-the-space but this is not enough or
does not work.

The same thing does happen when global-whitespace-mode is enabled
because then whitespace-buffer is called from a timer.
-- 
Stephan Stahl