Can you replicate in any other SAPI, I don't have fastcgi sapi  installed anywhere.

It doesn't make any difference with/without the patch for bug    #34191 (so I don't think it really broke anything)

Try with the shortest possible configure line:
# ./configure --disable-all --disable-cli --with-zlib

confirmed with ./configure --disable-all --disable-cli --with-zlib
both with cgi and fastcgi

good:
HTTP_ACCEPT_ENCODING=gzip php-cgi test.php |md5sum
826a819e34f7d57597d890de4176c265  -
bad:
HTTP_ACCEPT_ENCODING=gzip php-cgi test.php |md5sum
2e7dc64cd654b82f712b47b790dcbc12  -


(gdb) br 720
Breakpoint 2 at 0x805d6e5: file /home/xuefer/src/php4/ext/zlib/zlib.c, line 720.
(gdb) r
#0  php_do_deflate (str_length=2, p_buffer=0xbffff028, p_buffer_len=0xbffff02c, do_start=1 '\001', do_end=1 '\001')
    at /home/xuefer/src/php4/ext/zlib/zlib.c:720
(gdb) p outlen-zlib_globals.stream.avail_out
$8 = 10
(gdb) p buffer[outlen-zlib_globals.stream.avail_out]
$9 = 50 '2'
(gdb) step
724     *p_buffer = buffer;
(gdb) p buffer[outlen-zlib_globals.stream.avail_out]
$10 = 0 '\0'

the 2 is exactly what i see in httpwatch that get "lost" with the patch applied

test result by php->lighttpd->httpwatch is at:
ftp://211.92.88.40/pub/phpbug/test.php.response.txt
ftp://211.92.88.40/pub/phpbug/test.php.response-corrupted.txt

I've tried to replicate the bug using given methodology and removing the \0 line has absolutely no affect (md5 is the same) on the generated output.

i'd guess the "start_offset" is to avoid an extra erealloc. 
this bug should only happen when both do_start do_end is true (do_end=true && start_offset!=0)

 /* {{{ php_do_deflate
@@ -717,7 +718,7 @@
 
    if (do_end) {
        err = deflate(&ZLIBG(stream), Z_FINISH);
-       buffer[outlen - ZLIBG(stream).avail_out] = '\0';
+       buffer[outlen + start_offset - ZLIBG(stream).avail_out] = '\0';
    }

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.