Normally I'd pester the/a computer chess forum, but I couldn't register yet...
On the Matplus chess forum, the following construction task came up.
Both White and Black are on the brink of losing because the position is so sharp. Both players have only one way to draw the game. However, the seqeunce falls under th Nunn convention-each move is the only one that doesn't lose, but it has at least one legal alternative. They are denoted as “!” moves. What is the longest sequence of mutual "!" moves?
Here is an illustrating example:
[FEN "2K5/8/7k/6N1/3q1N2/8/8/8 w - - 0 1"]
1. Nf7+ Kh7 2. Ng5+ Kh6 3. Nf7+
Nalimov confirms that 1.Sf7+! Kh7 2.Sg5+! Kh6! is the only way to draw (by repetition), but since Kh7 is also an only legal move, this position is invalid.
I don't have Nunns "Secrets" series at hand, where surely some examples can be found. Neither can Imine a database for this question. Maybe you can?