 Lederman (2018b) uses the following scenario: Two contestants, Alice and Bob, observe the height of the mast of a toy sailboat (100 cm) that is subsequently replaced with a randomly selected sailboat whose mast may be more or less tall than 100 cm. As a matter of fact, the mast of the selected boat is 300 cm tall. It is therefore public information that the mast is taller than 100 cm. The ideal common knowledge principle above, along with assumptions about Alice and Bob’s visual systems and their publicity, would entail that Alice and Bob have common knowledge that the mast is taller than 100 cm, and yet Lederman’s argument shows that they do not. The main idea is that there is some degree of approximation in how humans perceive, among other things, heights. Thus, for Alice it is epistemically compatible with the mast looking 300 cm tall to her, that the mast looks somewhat shorter than 300 cm to Bob, say 299 cm. Also, Alice knows that if the mast looks 299 cm tall to Bob, then it is epistemically compatible for him that the mast looks 298 cm tall to Alice. Also, Bob knows that Alice knows that if the mast looks 298 cm tall to Alice, then it is epistemically compatible for her that the mast looks 297 cm tall to Bob. The reasoning can be repeated until there it is epistemically compatible for Alice and Bob that the mast is not taller than 100 cm, against the intuition that they have common knowledge that the mast is over 100 cm tall!