If someone sends a smiley face in Google Hangouts, and you copy this message and place it into a Confluence page, the editor will successfully render the smiley face as a span (not Confluence's native emoticon). When the user attempts to save the page, they will get the following message:
"Unable to communicate with server".

The only way to save the page is to remove the pasted external emoji/emoticon.

I'd expect Confluence to:

1. Not allow you to paste in characters that it won't be able to save
2. Convert these emojis to Confluence's native emoticons if there is an equivalent so the page can be saved