Preface: I haven't scientifically studied this or set up rigorous tests yet, but I have noticed it on 2 different outdoor shoots so it is at the very least a little bit repeatable. Both of those shoots were in very rural areas with absolutely no chance of electrical interference. Also, fresh batteries were used in both the trigger and receiver.
So the basic technical question is this: Is there any difference between pressing the 'Test' button on a CST and a shutter-release activated signal?
Here's what I noticed on the two shoots: When I got a fair distance away from the CSRB+ (attached to a WL1600 + Vagabond II, if that matters) I would find that the flash would stop firing when I pressed the shutter on my camera to take a picture, but every time I took the camera away from my face and pushed the 'Test' button it would fire with 100% reliability.
Shooting here, I probably tried this close to two dozen times (throughout this shoot I was walking further and further up the hill until I got out of range of the remotes. I'm not sure what the actual max distance was for the CST and CSRB+, but I was right at it for this shot):
'Test' would cause the flash to pop, but the camera wouldn't do it. Walking slightly closer to the flash/CSRB+ solved the problem completely; once I walked 20 or 30 feet closer to the flash the triggers went back to being perfectly reliable. That last bit is important, and it is consistent with what I experienced when this happened in another location. I'm not exactly sure of the distances, but when I was just at the maximum range the 'Test' would work reliably whereas an actual shutter-release activation of the remote wouldn't.
It was fairly annoying to take a shot only to have the flash not fire, and then press the 'Test' button from the same spot and see it pop just fine. The camera is a Canon 40D and it is 100% reliable with these remotes from closer distances.
I know you're going to be really skeptical of this Luap, and I don't blame you. I wouldn't take the time to write this out though if it didn't completely puzzle me, or if I didn't think that there was something to it.
Thanks!