Well, I jinxed myself there, didn't I?
No sooner had I posted about losing confidence in my Forerunner 405 than it freaks out on me.
I started the watch just like normal, heading out the door and up the hill toward the top of Keystone and the Peavine trails. At just past half a mile into the climb, I glanced at the watch and noticed it had gone back to the time readout screen. No problem - sometimes the cuff of a long sleeve shirt brushes it and sends it back. So I went back to the Training screen, to notice that there was a pop-up message saying "Hold reset to save activity." After some frantic button pressing, I found that, although the message remained on the screen, the timer and distance counter seemed to be working underneath it. That message stayed there for the rest of my run, until I got back to my computer and uploaded. This is what I ended up with:
Still not sure if I lost that first half mile or not - comparing it to other times I've done the same run, the mileage is pretty close.
A hard reset seems to have got the watch back to it's normal self.
This is what I'm talking about, though - I hate not being able to trust it. Starting to shop around at this point...