Those dreadful Dallas batteries buried within iCs. You have to break into the IC to kill the connection to the dead battery and solder on a new one without breaking the crystal. Luckily, these are socketed in Sun machines but I have done a couple where the Dallas battery was soldered onto the mainboard - one on a no-name German laptop and one IBM i think. Scary stuff as there is no room for error.
en.wikipedia.org
NT Sparc looks like a no-go. It's not even been confirmed that one made it out of a "proof of concept" build and if the Betaarchive doesn't have a copy, then nobody has leaked one yet.