I recently became aware of this thread in the mac_vim list.

In summary a user is complaining that suddenly his supplementary characters are rendered double width but should in fact be rendered single width.

I noticed the same for status characters used by a plugin in my setup.
Here's a screenshot of the issue:

After some bisecting I discovered the issue was introduced in 81a64f5 which implemented Emoji support.
The problem seems to be a result of a few new and changed, but now too generous, character ranges in src/mbyte.c.

Reverting only the changes to that file I get this:

I had a look at the unicode ranges for Emoji and - apart for the big Emoji block U+1F300+ - they are scattered across blocks in U+2300/U+2500/U+2600 und U+2700.

A proposed fix might be to check the output of runtime/unicode.vim and add the Emoji code points to the resulting doublewidth-table replacing the catch-all blocks that we have now.