Woah, really? That's quite a surprise. I've been using them in my hack and have seen no ill effects yet, but if these 'flags' at those values I used are actually wrong, then could that mean that there seriously are no safe flags besides the range you mentioned in one of your guides? If so, I'm going to have a lot of modifications to do with my hack when I get the time. I know you said there is no actual assurance that they are real, but to be on the safe side I'll take care of that post ASAP. Thanks for pointing this out to me.
Edit: I decided to simply delete it since it's otherwise useless, but I saved a copy just in case it somehow becomes relevant again at some point.
To start off, flags have a theoretical upper bound at 0x3FFF. However, from about 0x95F-ish up, there's no assurance that any of these flags exist. In your post, however, you say that flags around 0xAAAA are okay to use, which is not correct and I'd appreciate it if you fixed that before people actually use these flags in a hack. That "hidden flag" 0xC402 was probably just a bit that was set somewhere in memory that the flag reader read as a flag - but that doesn't mean it actually is.