Macbook pro 13" mid-2012 not turning on, no battery leds (battery ok)
Not sure how many other people will encounter this, but the previous owner of this MacBook made an error while replacing either the battery status led part, or the battery itself. 

When I once again tried to re-seat the battery, I saw that the cable to the led indicator was damaged. Apparently it had shifted somehow and got damaged by the outside screw of the battery. After I disconnected the cable (and eventually snipped off the damaged part of the indicator to prevent it hanging loose), the MacBook booted and is running and charging it's battery just  fine. So, if the cable in the picture is damaged and shortcircuits to the case, your Mac may not boot or do anything at all.