Sort of what I was wondering.
The Brits, back in 2019, did not take delivery of the MODE 5 transponder for their E-3 AWACS from Telephonics and cancelled the contract. I do not know what the issue was, but it left them without MODE 5 capability less than three months from implementation. I do not know if there were other users who did the same, with other platforms.
When aircraft fly in formation only one has an active transponder to prevent garble. I wonder if each thought that the other guy had his on, or if both were on and the destroyer had gotten garbled nearly simultaneous responses. The MODE 5 interrogation is encrypted but distinctive. If an enemy cannot give a valid reply, they could very well give a simulated garbled reply, hoping to confuse the interrogator.
I have talked to old E-3 radar operators, and they claim you can pretty much sort out players without using IFF, IFF is just a form of confirmation. They had experience in Iraq during the current shitshow and claimed they could tell the players without a scorecard to read the jersey numbers. They get a feel. But a valid IFF reply is the next best thing to divine word from the Holy Ghost.