A few weeks ago, somebody else pointed out this exact same bug to me. Actually, it's a bug in Zlib, not PNGOUT. As you probably know, just about every program in existence that has some kind of ZIP or PNG support (other than my stuff of course : ) uses Zlib. I even wrote Mark Adler (Zlib co-author) about it. Here's what he had to say:
I believe that the bug you are experiencing is only in zlib 1.2.1, and it is fixed in zlib 1.2.2, which you can find here:
http://www.zlib.net/
Zlib 1.2.2 was released about a month ago. I could easily do a hack in PNGOUT to make it produce files that are compatible with your buggy software, but I don't think that's the right thing to do. Instead, you should be nagging Jason Summers (Tweakpng author) to upgrade his code to the latest version of zlib.