I've had the opposite to the Avocets above, whereby a ringer has said the combination didn't exist in any of the schemes. Tthe ringer suggested one of the colour rings may have faded (from green to white) hence my confusion with recognising the correct colour. A check with the new suggested colour combination also failed to find a match with any of the schemes.
With Avocets too, I had a quick reply from a French scheme once, but they couldn't give me full details of the bird I saw because it had a ring below the knee that I didn't see/report - not often that you see below the knee of a wader that spends most of its time in the water is it?! and makes me question the wisdom of doing that!
I also heard of someone ringing shorebirds in Dorset a few years back who wasn't submitting any data to the BTO, just a rumour, but from a reliable ringer source, I wonder if this accounts for some ring reports never getting a response. I also wonder how 'within the law' it is.