While implemented Amended-Needham-Schroeder, I accidentally wrote the following very subtle bug:

Intruder
    var M : Msg .
    var N : Nonce .
    M, N <=> M ; N .

rather than the intended

Intruder
    vars M M1 : Msg .

    M, M1 <=> M ; M1 .


I only realized this was a bug when I was failing to find the expected attack, and I looked at the generated Maude file, in which I
saw the very helpful: 
      -M:Msg,
      -N:Nonce,
      +(M:Msg ; N:Nonce) 

Catching this kind of bug is tricky, because