The thread is still ongoing, but basically, in the midst of his mad scientist mania while Frisk was dead, Sans tried to recreate some of the experiments that he and Gaster did to first give him his powers, in the hopes of making himself stronger. On the one hand, it worked a little - enough that he's not dead after everything he put himself through (and probably the only reason he's currently able to operate on fractional HP).

But on the other hand, at least in my backstory for this specific Sans, this won't be the first time he's transformed like this. Gaster was able to change him back last time and get him stable. Gaster isn't here right now! So that will eventually be a thing that happens, since he basically overloaded himself and that power's gonna have to go somewhere.

Sans is aware that this is a possibility, but not really clear on anything he can do about it, so he's just kind of trying to keep the possibility in the back of his head. Right where dreams usually get their fuel!