I have a sign that was given to me by another player character yesterday. The sign was blank and able to be written on to begin with. I think I entered the writing menu, but exited out because I wanted to think about what to put on it + know how much space I had to work with. Now I just have a blank sign that I can't write on at all. I didn't realize at the time that signs couldn't be edited. 

I feel like signs should work more like paper, where text can be added but not removed (though this could run the risk of vandalism), or that there should be some kind of grace period (maybe 24 hours in real time?) where a sign can still be written on.