I perhaps should say that this is not a problem with all Copal shutters to my knowledge but I was able to reproduce this problem on three of mine. It happened this way. I was out shooting about a week ago and had a light reading that required a shutter speed of ¼ second for my desired effect.
I was shooting an outdoor scene with my prime subject being a female friend. In all of the settings to consider such as light direction, background, posing and everything else I guess made me a bit casual about setting the shutter speed indicator. This is the first time this has happened to me and I was quite surprised to discover that this is normal.
What happened is that I had the indicator slightly off the click setting for ¼ and was exercising the shutter a few times while talking with my female model and attempting to put her at ease. When everything seemed good I began shooting Tri-X and everything seemed to have gone off without a hitch.
The next day in the darkroom I discovered that most of negatives were considerably under exposed to the point of being almost useless. I thought about the problem for a few minutes and concluded that the shutter was the culprit. I thought this strange as I use my Nikkor 300M often as I do with my Nikkor-W 210. The problem turned out to be that if the shutter is not set exactly at the indentation for ¼ then the shutter behaves more like 1/125 which obviously greatly under exposes the film. I proved this at home while playing with three of my Copal shutters. Just thought I might pass this along before somebody else as green as me gets bit by this.