Someone will probably say this isn't a bug, it's intentional, but a "feature" this disruptive counts as a bug in my book, regardless of whether it was intended or not.
The problem, of course, it with mouselook and the ability to look at your feet. If you're standing at the moment, go into mouselook and look at your shoes. Works great!
Now, sit down (on some prim without some fancy animation, a newly created 0.5m square box, for example) and try to do it.
Can't get the camera to aim at your feet, can you? Why, for the sakes of all the gods, are we now allowed to look at our feet, or look into our laps, when we're sitting? What useful purpose does this serve? Prevent us from spending too much on napkins when we can't see the food that fell in our laps?
Here's the fun part. Make a mouselook vehicle with a decoupled camera. Point your nose into the sky and fly straight up. Now, try to get back to where you started. Only way to do it is by "tacking". You would think flying straight down would be the easiest way to fly. Heck, even rocks can do it. But this vehicle can't, because you can't point your nose down towards the ground. And this time, since the vehicle is rotating, it's not like there's any worry that you might see your feet (still don't know why that's a feature, but let's assume for the moment that it's intentional that we can't see our feet while sitting). Since the vehicle rotates just fine, you ought to be able point your nose straight down, you'd still be in no danger of being exposed to the dreadful sight of sitting feet. But no, even if not being able to see your feet is a not a bug, this still counts as one, because it's attempting to prevent you from seeing your feet even when you'd be in no danger of seeing them anyhow. Whiskey Tango Foxtrot, over.
</rant>
Question: is there some work around for this problem? Or is camera decoupling just not usable for any vehicle that needs to be able to point its nose downwards more than 40 degrees? I tried rotating the sit target, that leads to even stranger behavior (e.g. you can point your nose down 80 degrees when flying east, but not at all when flying west -- perhaps there's a secret base to the west They don't want us to see
).