As discussed in https://moodle.org/mod/forum/discuss.php?d=278682, while copying text from Libreoffice Writer to Atto text editor, CSS code is getting displayed on top.
Here are more details/steps:
OS: Ubuntu 14.04.1 LTS
Step 1: Insert/Add some data in Libreoffice Writer.
Step 2: Save file (either in .docx, .odt) and close.
Step 3: Reopen file.
Step 4: Now, copy and paste content from that Libreoffice file to Atto text editor in Moodle (any activity or resource can be used which have atto text editor).
Step 5: Click save and display.
Step 6: CSS junk will appear at the top.
Above problem however does not exist when copying & pasting data from webpage to Atto Text editor.
Bug also successfully reproduced on demo.moodle.net