I'm getting some screwy indentation behavior from C mode.  From a
current bzr checkout/install of emacs, try the following recipe: 

emacs -Q foo.c                 | starts emacs on foo.c (included below)
C-u 7 C-n                      | move down 7 lines
TAB                            | indent, nothing changes (good)
C-p C-p                        | move up 2 lines
TAB C-n TAB C-n                | indent 2 lines, nothing changes (good)
TAB                            | line unindents by 2 (bad)
TAB                            | line re-indents (good, but strange)
TAB                            | no change this time

And here's another recipe for weirdness:

emacs -Q foo.c                 | starts emacs on foo.c (included below)
C-x h                          | select buffer
C-M-\                          | indent region everything from line 7
                               |   down to the while (0) unindents 2
C-u 5 C-n                      | move down to the do {
C-x ( TAB C-n C-x )            | define a macro to indent and move down
                               |   a line
C-x e e e e e e                | Indent each line, no change

Here's foo.c