http://imgur.com/a/k84hF
As you can see, the heatsinks will not drop in any slot.
EDIT: The bug is bigger! No equipment at all could be added to my mech at the time of the screenshots above. However, I found that dragging a piece of equipment on to the slots that are whited out seemed to equip it. I'll try to recreate as best I can.
Edited by Treysef, 07 May 2015 - 12:30 PM.