I put new rings and a new scope on one of my most accurate rifles about a month ago. At first it shot well but then last time out by the end of the session the groups started opening up so I pulled the scope off.

The front base (Talley LW 1-piece) was loose. First time I've had one come loose so I'm thinking I must've had a brain fart and not tightened the bases all the way when I put them on. I'm usually careful about that, use a torque wrench, etc. This is not a hard kicker either, .260 Rem.

People always say the rings/bases are the first thing to check out when groups open up but like I said I've never had it happen before.

So my question is, how common is this? Has anyone had this happen when you're sure you tightened the bases right, or is it almost always operator error?

I've never had them loosen when properly torqued. I also degrease the threads, both male and female, with isopropyl before installing.

Blue loc-tite is your friend, and as Pahntr said, it's a good idea to carefully degrease the threads first. Not only does the blue loctite give you some security against loosening under vibration, it acts as a lubricant when installing the screws for a much truer torque reading. It's kind of like the lap or not lap fans when it comes to thread locker but I've had excellent results with it.
John

Check the screw length on the foremost screw. It is common for it to be a little long and bottom out (appearing to be tight) before it snugs the base to the receiver.

Degrease and loctite everything on ours but note the screw length issue, thats common enough to casue head aches.

That was the first thing I checked. I should've known they'd be OK though, it's not like Mr. Forbes to leave that kind of thing to chance. Both screws were loose, I'm pretty sure it was operator error.

I've had it happen twice, recently with talley LW which i used no loctite, happened at around 1k rounds of 7wby mag.I usually put a dab of purple or blue on the screws, must have had a brain fart on that one.

The other was on a 30-378 with EAW mounts,and happened while on a hunt. its been an SOB even with locktite..I use red on it now ha.

I had it happen once with Talley light weights on the front only. Screws were the correct length.