I've noticed that on occasion, in a sim with every bot being mutation-disabled, a spreading virus can sometimes mutate, always in my experience being the first bp (which, also, is always a cond, don't know which it is connected to). This is changed to a 0, which makes me believe that the word cond is somehow being corrupted and the interpreted is reading the unknown word as a zero. I am using the latest buddy drop, although I recall this happening in earlier versions. I haven't tested this with the sim-wide mutations disabled. Furthermore, the only mutation registered in the mutations dialogue area is the viral infection itself, so I don't think actually mutation code is being run (and if it was, it wouldn't happen in the same place and way every time.)
Tell me if you need more info.