This is a bug between TinyMCE and WebKit based browsers. This is due to the fact that the style parameter is interpreted as a html attribute (ie img style=”float: right”). If you go to Text mode and copy and paste all will be fine. As long as you stay away of naming your shortcodes parameters as HTML attributes all should be fine.
PS: I have found this in the Paste plugin for TinyMCE 🙂 :
// Since WebKit/Chrome might clone the paste bin when pasting
// for example: <img style="float: right"> we need to check if any of them contains some useful html.
// TODO: Man o man is this ugly. WebKit is the new IE! Remove this if they ever fix it!