If you connect a display to your MiSTer via HDMI, here's some good summary info and advice.
Apparently, the DE-10 Nano's HDMI port is not electrically buffered or isolated from the rest of the main board in the ways you (and other consumer electronics devices) would normally expect. For example, apparently its 5V line is tied directly to the main board's 5V rail, such that if 5V is present on that pin of the HDMI port, then some portions of MiSTer's hardware can appear to "turn on" even while the MiSTer itself is not powered up or even connected to power! As such, MiSTer can exhibit a myriad of weird issues while hooked up to a display system via HDMI, especially if you are using long cables or running through an HDMI switch, AV receiver, HDMI capture device, or any kind of standard HDMI adapter/converter:
decreased SDRAM stability
display device randomly losing sync/signal from the MiSTer or taking an excessively long time to lock in sync after a resolution mode change
MiSTer's cooling fan spinning, or its lights lighting up, even while MiSTer is powered completely off
HDMI switch refusing to change inputs and/or randomly changing inputs while you're not using MiSTer and it is powered off
USB dongles for wifi and/or Bluetooth not coming up and working reliably on every boot
...and possibly even other weird symptoms
What I have found is that the following inexpensive HDMI booster gadget works to effectively isolate/buffer MiSTer's HDMI output:
And it seems to work without supplying it any external power over its microSD port (i.e. the power produced by MiSTer's HDMI port appears to be enough to make it work). By installing one of these between my MiSTer's HDMI output and the HDMI switch I used in my home theater system, I was able to cure a combination of the above issues that I'd been beating my head against for months.
So, if you're using HDMI at all, it's probably just a really good idea to get one of these little booster devices and install it, to electrically isolate things. And if you're putting MiSTer inside of a larger case of some kind, it's probably an even better idea to install one of these inside of the case, so that the HDMI port exposed on the outside of the case is already electrically isolated.