How can I reboot into recovery from fastboot?
(Self answering here because I only found this answer after hours of dredging through 200 page threads of forum posts.)

There are two non-standard but semi-prevalent methods that may work depending on your make and model (and why recovery isn't working in the first place).


Some phones actually respond to fastboot reboot recovery. This is quite natural given adb reboot recovery, but unfortunately is non-standard. I found lots of people suggesting this method so I offer it here in the hopes that it works for some, but it didn't for me.
Some phones will actually let you book into an image via fastboot without flashing it. This will allow you to load a recovery image without needing to reboot the phone at all. You need a copy of the recovery image on your computer, but running fastboot boot image.img should do the trick. I was able to load up TWRP using this method.