Not sure if it happens with every merc. I was playing with Bushwhacker when it happened (and it kept happening in every game played).
The bug: If you become incapacitated while repairing the EV / APC you can continue repairing it, if you keep the F key pressed (or the corresponding key in your machine).
It happens at any point of the EV path, not just at the beginning.
Here is a video of the occurrence;
[video=youtube_share;hhuxu7oytvk]http://youtu.be/hhuxu7oytvk[/video]