no, i think in this case: he accept the nick that full friend slots w/o check the error notice.

Actually, it has happened once before, so this time I have checked that :
1) the request was new (Pending list was empty before)
2) after accepting, he was in my Confirmed list as a new ally with the same name
3) he has stayed in the list for approx 10min
4) then he left and was not in my list anymore

Well, it's not really important, it's just strange...
And it is really annoying if there is bots in this game.