When I was trying to produce contours for my homage to the old 1/2" maps I discovered that there are "holes" in the SRTM altitude data. The software I was using doesn't seem to handle them very well, and they produced some strange effects, but those were quite localised. As far as I can see, all the problems were well away from the main areas of the UK that I was working with. So I added this to the list of things that would be interesting to explore later, and I ignored them.
Then today I came across this on the Ordnance Survey blog.
It's about producing a list of holes in Britain, as an addition to the existing lists of mountain peaks such as the Munros. My immediate reaction was that this must be related to the same problem. It isn't of course, but the video was worth a view anyway.