The recurring annoyance: a headless box in the cupboard that, once or twice a year, decides to sit at a BIOS prompt or hang before the network comes up. SSH is no use when the machine hasn't booted far enough to bring up sshd. So I'd drag a monitor and keyboard down the hall, crouch on the floor, fix it in ten seconds, and carry everything back. Daft.
A KVM over IP solves it, and the commercial ones cost a small fortune. A Raspberry Pi does the same job for the price of a takeaway. PiKVM is the project that makes it trivial: a prebuilt image, a cheap HDMI capture dongle for the video, and the Pi's USB gadget mode pretending to be a keyboard and mouse to the target. You get the screen and full control in a browser, from boot, BIOS included.
The fiddly bits are real but small. You want a Pi 4 so the USB-C port can act as a gadget device rather than just taking power, which means feeding power through the GPIO pins instead. The HDMI capture stick wants to be a known-good one, since the cheapest no-name dongles drop frames or refuse certain resolutions. And ATX power control (actually pressing the power button remotely) needs a couple of wires onto the motherboard header, which I haven't bothered with yet.
Even without the power-button wiring it's already paid for itself. Last week the box wedged at a filesystem check on boot. I opened a browser, watched it sit at the prompt, typed the answer, and never left my desk. That's exactly the faff it was meant to kill.