An ampersand in a link sometimes gets converted... [:S]
Can you provide a few more details. Are you using the quick reply or the full editor? Is it a URL entered with the link tool or did you simply paste this in the editor?
this is a test
http://macromonster.com/index.php?mod=descr&id_desc=166&
Several pasting scenarios have been shown in a video that I sent to Gerard - direct - just now.