BIT Counting: C programming problem

[Doug Gwyn]

In article <11025 at rpp386.Dallas.TX.US> jfh at rpp386.Dallas.TX.US (John F. Haugh II) writes:
-That wasn't what he wrote, he wrote
-	num &= num - 1 , count += 1;
-		       ^-- oops.

Excuse my asking, but what's supposed to be the problem with that?