My work HP, however, is regularly scorching hot when I get home as it's been on in my backpack the whole time despite me shutting the lid and despite the instruction to 'go to sleep when the lid is closed'. How there's not some sort of thermal cutoff I do not know, but I know for a fact that there is not.
Edit: by 'regularly' I mean a few times a year. Still far, far too many.
My former notebook (Acer Timeline 1810TZ) used to wake up from a press on any keyboard button, which I was unable to change. The issue here is that with sufficient pressure (e.g. when I was running with the device in a messenger bag), the lid would bend enough to actually press a key and wake up the machine.
My hackish solution was a script that would automatically put the laptop back to sleep a few seconds after waking up, which I then manually killed after intentionally waking the device.
Unfortunately, it's close to 100 degrees. It's only there to prevent damage to the CPU, not the rest of the system (or your back). It also isn't guaranteed to shut down the laptop, just the CPU.
So it really depends on how well the motherboard manufacturer did their job.