Seem on mac, there isn’t a lot options for the download manager, so one of my choice is Firefox + DownThemAll. Even though my primary browser is Google Chrome.
Today I face something new, my download files all get corrupted. Just by doing the MD5 checksum, I realize the problem is within the DownThemAll.
After digging down, the problem is because 1 or 2 addons which is YSlow and Firebug. Just disable both and immediately all the new downloads is normal again.