Well, it happened to me too. Lost a nut.
 

Yesterday I rode my bike to breakfast and the first thing I noticed was how it was vibrating differently than usual. Mind you, I just hit 500 miles on my bike but I already got a good sense of how its supposed to sound and feel when running normal.

After breakfast starting it up again it just didnt sound right. I had an extra little metal pinging and exhaust "put put".

When I got it home and put it on the stand I did a onceover on the bike. Wouldnt you know, my exhaust nut on the back header was missing. The flange was rattling and I had a slight exhaust leak.

Luckily I had an extra nut from parts I had already removed and tightened it back down.

Just another thread to make sure everyone is checking the nuts on their exhaust. Guess you cant have enough reminders that it can happen to anyone at anytime.

I replaced my pipes a few months back and the flange nut torque was good when I rechecked the torque at 50 miles. I just finished rebuilding my rear caliper and to get the rear axle out, I had to remove the top muffler at the rear header pipe. When I was putting it all back together, I noticed that the front header pipe was a little loose. I checked and found that all four header nuts were substantially below torque. I will make a point to do more periodic checks from now on.

Preventative maintenance is the best kind of maintenance.

You probably still may have the incorrect nut on there. It should be a harden, locking style nut (not nylock either..the kind with the crimp at the top). It's too hot for Loctite, and you are pulling up on a split ring that is around a thin pipe flange up against a crush SS wire woven gasket ring. The lock nut keeps it tight. Even if it got loose, it still should not have vibrated off. That tells me someone has already been there and replaced the pipes with cheap hardware store fastners.

Well, I did replace the pipes the moment I brought the bike home with 8 miles on it. However, I didnt use "cheap hardware store fastners", I used the OEM nuts that were on the bike. I also replaced it with another OEM nut that is supposed to be on there.

Those nuts can take a few heat cycles to set. I've R&R'd exhaust multiple times without issue. Take a small socket wrench, 3" extension, and the universal joint socket with you each time you go out and see if you can tighten the nuts without much effort. Only do this on a cold bike or you risk snapping the studs. I've found after 2-3 heat cycles they won't budge. After that, I just count the visible threads on the stud and do a quick visual check every now and then.

If it's a concern you can buy 12-point self locking nuts from ARP, but you should not need them.