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In the past few weeks since the Steam Machine has reached early users, relatively few outright issues have emerged - beyond a mistaken case of flipped error codes causing a user to understandably mistake memory training for GPU failure. However, some users have noted that their Steam Machine was showing temperature warnings, and after some back-and-forth with Valve, it seems like a fix is on the way.
The Steam Machine is meant to turn on its high temperature warning (a full red band on the LED that runs the width of the front case) at 95°C for the CPU and 90°C for the GPU. That makes a lot of sense, as both components are quite happy to dynamically raise clocks to wring out some extra performance, as long as power limits are still being observed.
However, it looks like the issue reported by /u/Pure-Outcome-5977 is that the temperature warning is coming on before those temperatures are actually reached, given that the user's screenshots show CPU/GPU temps at 81°C and 75°C respectively. I'd also add that there could also be a difference between the sensor being used to control the LED (eg hotspot temperature) and the sensor being polled to show CPU/GPU temperatures in the performance overlay (eg core temperature).
The Steam Support rep confirms that, even at 90 or 95°C, the Steam Machine won't come to any harm, and it will start to thermal throttle at 100°C - dynamically lowering clock speeds to ensure the silicon can cool down for a period. It will also shut down if its temperature exceeds that 100°C threshold for too long, which is normally sufficient to prevent any damage.
Note that some outlets are reporting that Valve is set to change the warning threshold to 100°C on both CPU and GPU, but while the rep does say this, this wouldn't make any sense - as the issue is that the warning seems to be occurring well below the planned 90/95°C thresholds. In addition, CPUs would start throttling at this point, so the warning would only be visible for one tick before the temperature dropped again, if that.
Have you seen any error codes on your Steam Machine too far? Ours has survived a couple of UK heatwaves at this point, but it is free-standing which aids airflow - one packed into a TV cabinet could get toastier much more easily. Let me know in the comments below.