Strange pedantic error message

[James Montgomerie]

When compiling with -pedantic (which I know I'm not supposed to do, I just
thought it would be interesting - I don't usually use it), I often get
the following warning:

warning: ANSI C forbids conditional expr with only one void side

I've done Google and Deja searches for the message, but can't find it.
What does it mean?  The thing that seens obvious from my code and the
wording of the error message is that 'if's without 'elses' are not allowed
by ANSI C.  I find this hard to believe, so I thought I'd ask for
clarification.

Thanks,
Jamie.