I have the exact same error in tekkit 3.1.2, both on singleplayer and my own multiplayer server (using hamachi). The nano saber, while off, behaves exactly as expected. Once switched on, it cycles through on, off, equipping and unequipping in no apparent order. I would attach a screenshot, but it would not properly demonstrate the error 
. Once, I managed to get the error to not occur, but I have no idea how . Also, if multiMC works as I think, the same effect could be achieved by starting minecraft through the command line, which I have tried, yielding no error message.
EDIT: Even while appearing to be off, the sword is still actually on, and still one-hits creepers, but the graphical problem is still very annoying, and a fix would be greatly appreciated.
FIX!!!!!: Go through a nether portal with the glitching sword on, and come back immediately. Fixed. EDIT: This fix is not entirely reliable, as I was able to implement it twice before it stopped working, including the time above when I didn't realize that it was the nether portal that had fixed the sword. I believe that the sword needs to appear on at the moment the travel completes, though this is nearly impossible to practically achieve.