This is the situation Moosezilla (who is one of the CCM-prebetatesters) reported some posts above and proved it with a save-file. I made the screenshots from the save-file when the same action (as always) happened again:
In CCM mountains, volcanoes and marshes are not allowed for landunits with the wheeled flag. Most landunits in CCM have the wheeled flag. So the Japanese Asiatic chariot has the wheeled flag, the Japanese settler and the Russian missionary on top of the mountain don´t have the wheeled-flag.
The Russian missionary of the human player finished its turn on the mountain. In the neighbouring tile there are the Japanese chariot and the Japanese settler. This should be an ideal position for an attack of the Russian missionary, as the chariot can´t move on the mountain and the settler can´t attack another unit.
But what happened? The chariot, that escorts the settler and shouldn´t be able to enter the mountain (that has no road on it), attacked the Russian missionary located on the mountain and killed that unit. The result was made visible for the screenshot by another Russian missionary nearby. After that attack both Japanese units keep standing before the mountain until the end of time.
The conflict of units with the wheeled-flag and escorted units without such a flag caused a lot of freezes in CCM (I made a report about such a situation with ships many posts earlier). That this conflict even leads to a situation, that an escorting unit attacks on a terrain, that it shouldn´t be allowed to enter, was something new for me.