On this question, I'm seeing bananas in two of the comments:
Am I going bananas?
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Those are instances of U+1F34C ʙᴀɴᴀɴᴀ. I use them as a filler in comments. I used to use U+2063 ɪɴᴠɪsɪʙʟᴇ sᴇᴘᴀʀᴀᴛᴏʀ, but since some people complained they were seeing the invisible separators as squares, I switched to bananas. They also have the advantage of being represented in UTF-16 as surrogate pairs, and thus each counts as 2 for the 15-char limit.
They look like this on my system (Windows 7, Firefox):
Mac OS ships with some fonts with Apple extensions to OpenType that allow the use of colours in glyphs, and that's why you see them flashy like that.
It's shorthand for "flag this comment as noise so that it can be removed".
The banana itself is a reference to a famous theorem.