The system isn't perfect. A few years ago I was at cruise in a C150 below San Jose. ATC suddenly thought I had a 7700 code. My code wasn't even close. It was some kind of glitch on their end. I think the codes are assigned such that they aren't close to the special codes so that when you change codes you shouldn't hit them, at least if you careful.
Fruit? (False returns unsynchronized in time?) Everyone squawks on 1090 MHz, using the same pulse coding. An aircraft will be interogated by several radars at any one time. The radar receiver tries to sort out overlapping replies and generally gets everything right, but there's a small (like 1%) chance of not detecting an aircraft on any one scan and an even smaller probability of making a decoding error.
But such a condition would not persist for several scans, so that is not what is happening here.