Compressed SWF files contain a version byte. At the moment, it is checked if this version byte is between 0 and 10:

if ((swf_version > 0) && (swf_version < 10)) {

Unfortunately, the highest possible SWF version is 32 as of Mar 10, 2016 (list at Adobe, StackOverflow question) and it's very likely to proceed further.

As the 3 preceding bytes (CWS) don't leave too much room for false positives, the best solution is to just ignore the version flag.