This is a glitch I've discovered unintentionally. I don't know if this thread is still active, but here it goes:
I was helping a random guild tower a few days ago and I decided to leave after some time of helping. To save time, I kicked myself from the guild and tapped back really fast until I closed my profile before the 'You have successfully left the guild!' message popped up (this was achieved by lagging). Then I still had the guild chat button and I was still receiving messages from guild chat. I was confused and I tried sending a guild chat message asking the members if they received my messages, and they said they did. I opened up the guild roster and checked to see if I was still on there but I wasn't. So I could pretty much spam and send guild chat messages without having the guild tag.
How would this be abused? You can pretty much spam guild chat without ever being kicked. Unless you set another guild tag or log off then back on again. I haven't tried entering the tower and killing the guild holding the fort. I've seen it happen to someone else a while ago. I've tried this on two guild tags (both were holding towers before I joined and left). Hope this was of help!