A friend pointed out this sentence from the CD-ROM of the Cambridge Advanced Learners' Dictionary:
Understand (KNOW) 5 understand one another/each other When two people understand one another, they both know what the other means and wants and they have an agreement: Both sides must try to understand one another, to recognise each others' rights, feelings and beliefs. See also understanding (AGREEMENT).
Can you spot the mistake? The apostrophe is in the wrong place. According to www.yourdictionary.com:
The possessive forms of each other and one another are written each other's
and one another's: The boys wore each other's (not each others') coats. They had
forgotten one another's (not one anothers') names.
Tut tut, Cambridge.