I have never seen a tapered groove for a snap ring. I have seen semi-circular grooves for formed wire retainers.
The main problem that appears to be this case is if the person installing the ring thinks that just because they have pushed the ring into full flat contact with the item being retained that the ring must be in the groove. Given the spring load the ring will be wedged into the bore and appear to be secure, but if one doesn't look to see then this situation can certainly occur.