When using a spyglass with a carved pumpkin equipped on the player's head, the spyglass level does not match the carved pumpkin's eye level, even though that is how the player looks through the spyglass. This is because the carved pumpkin's eye level is above the player's actual eye level.

Other mob heads have this same effect, but are highly likely to not be bugs. This could be considered a bug due to the overlay and the resolution of MC-204159, which points to the fact that the player would logically be putting the spyglass through the carved pumpkin's eye-hole.