I used to work for An Post and officially I was told to write 'it is the policy of An Post' rather than 'An Post's policy' as you couldn't mix languages. I also wrote a little note for the webmaster as to when to use it's, its' and its.
In school we weren't taught english grammar but when learning French the teacher started from there.
My pet peeve is tennants...you know those beer cans that rent property...
And recently the Oxford comma. I really myst get out more