Yep it is a known bug.
If you need to Copy/Paste, try copying into the Mac equivalent of Windows' Notepad first, then copy that and paste into a post. (Sorry not sure what Mac's Text Editor is called).
Also, when typing, turn off the post box's own "Text Editor", the icon on the far right in the formatting toolbar, next to the smiley.
Mark
For example, taken from a post to Mac ----Peripherals made a few minutes ago:
<span style="color: rgb(66, 66, 68); font-family: Helvetica, Arial, sans-serif; font-size: 12px; "><span id="INSERTION_MARKER">
<span id="INSERTION_MARKER">"My text<font color="#424244" face="Helvetica, Arial, sans-serif"><span style="font-size: 12px;"> </font><font color="#424244" face="Helvetica, Arial, sans-serif"><span style="font-size: 12px;"><span style="border-collapse: collapse; color: rgb(51, 51, 51); font-family: 'Lucida Grande', 'Lucida Sans Unicode', Helvetica, Arial, Verdana, sans-serif; ">More my text."</font>
<span style="color: rgb(66, 66, 68); font-family: Helvetica, Arial, sans-serif; font-size: 12px; ">
<span style="color: rgb(66, 66, 68); font-family: Helvetica, Arial, sans-serif; font-size: 12px; ">It appears in several other peoples' posts as well, and makes a post more difficult to read.
<span style="color: rgb(66, 66, 68); font-family: Helvetica, Arial, sans-serif; font-size: 12px; ">
<span style="color: rgb(66, 66, 68); font-family: Helvetica, Arial, sans-serif; font-size: 12px; ">and when I Preview this post, more HTML is added to it!

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