The thing that annoyed me recently is that if I cut & paste a single character in Google Mail, it'll "helpfully" surround it with spaces when pasted. Other applications like Firefox or MS Word don't have this behaviour.
I just tested it, and the GMail behaviour is wonderfully inconsistent:
Copying from GMail to to plain text (e.g.: a text editor) will result in a single character.
Copying from plain text into GMail will result in no superfluous spaces.
Copying from GMail to GMail or to any "rich text" editor will add the extra spaces.
So basically I'm copying plain-text, but GMail is abusing rich-text formatting to add "magic" behaviour someone at Google assumed is beneficial, but is just confusing and arbitrary.
PS: Similarly, GMail's spell checker now "fights" with the Firefox spellchecker and the result is that neither wins and obvious typos aren't resolved unless I turn checking off and back on a couple of times.