This problem appears w/Chrome when its used to input text in Umbraco using TinyMCE. Other have reported the problem.
The problem does not occur when you use the http://www.tinymce.com/tryit/full.php tester in the same version of Chrome so may be because Umbraco is using an older version of Tiny.
See:
http://our.umbraco.org/forum/core/general/24303-Chrome,-TinyMCE,-and-Line-Breaks
I can confirm this - for some reason Chrome actually wraps the entire pasted text in a <pre> tag so you get this mark-up:
Glad someone else has confirmed it - thanks Dan.
But if you go to http://www.tinymce.com/tryit/full.php, it doesn't happen - so I don't think it's something to do with Chrome entirely. I think the version of TinyMCE in Umbraco does something which causes it to happen. So there must be a way around it.
Anyone have any ideas?