Coupling coaches with British Standard and Pullman gangways was very common. All GW and LMS coaches had BS gangways and, as the proportion of BR Standard coaches with Pullman gangways increased, inevitably such connections had to be made - most trains were far from uniform and some LMS and at least one GW coaches lasted long enough to be repainted blue & grey.. In those days I never heard of the clip 'snagging' but people did check.
An incident occurred on another 'heritage railway' where an LNER Pacific (with buckeye corridor tender) was coupled to a matching set of coaches. Unfortunately, the saddle was not lifted from the buffer stem of the adjacent coach. It was quite difficult to couple up, though this was eventually achieved by some straining; but, unsurprisingly, extremely difficult to separate.
The number of incidents (some fatal) on these lines that have attracted the attention of RAIB is interesting, and incidents like this one do suggest enthusiasm is trumping expertise.