I encountered something interesting today. When I maxed out the CPU with 8 threads of "yes > /dev/null &" while on battery, the macbook pro went into suspend mode after a couple minutes. When I tried to turn it on, I got this symbol. It refuses to boot up on battery. When I plug it in, the laptop starts up just fine with 30% battery... I was able to reproduce this a couple times. EDIT: Looks like my battery just gave out on me.