I hadn't heard of slippage before, but according to the technical advice from thread company American and Efird, if "stress is applied to a seam, the yarns in the fabric slip out of the stitching causing an open seam" (see here for the PDF)
It happens if the fabric has relatively loose weave (tick for the wool challis), the seam allowance is scant (tick for my sewing of the sleeves on this shirt) and there is a single row of stitching (more or less the case with the shirt, even though the seam was bound with a bias strip). Previous shirts in cotton, polyester and linen have never given this problem, so it's not strongly related to the pattern, except that this part of the armscye seam will always be under a little bit of tension when the wearer moves her shoulders.
If I had to have seam slippage on this shirt, it couldn’t have come at a better time, as I was at a sewing class at the wonderful Cloth Shop in Ivanhoe (Victoria Australia- here's a link to them) and could get instant sympathy and advice. Following the advice, I unpicked the binding from the worst affected seam and then unpicked the seam where the slippage was obvious. I cut a piece of high quality soft and lightweight fusible to the shape of the back armscye and ironed it on very carefully. I used just the tip of the iron and positioned the interfacing so that it slightly overlapped the edge of the body fabric where the seam had opened. Then I sewed up the seam, making it slightly deeper at the stress point. I cut a new bias binding to replace the chewed up original (which was also now too narrow to cover the deeper portion of seam) and re-did the binding. I then stitched a new seam ever so slightly deeper all round so there was a double row of stitching.
On the other armysce, where the slippage was not so dramatic, I just unpicked the binding enough to enable me to apply a shaped strip of fusible over the back and slightly into the seam allowance where the slippage was visible, then restitched the binding and double stitched the seam.
Here's a view of the repair on the inside
Now, to prevent any possibility of slippage on the silk shirt that is my next project, I will apply a self-fabric interfacing to both armscyes, front and back. I'll be sure to leave a bigger seam allowance for insurance.
Wish me luck!