It is really east to add on a pre, re, dis, or un, to the front end or an ed, er, est, or ing to the back end of the word. Even when it is not exactly common use it can still be grammatically correct and therefore legal and playable. When not, it is worth a challenge.
For instance, I dropped "untrade" onto the board the other night and then sold it pretty hard. It was an 87 point bomb, but only about the third round. The choice was made to let it go. Turns out it is not playable.
Apparently, "prego" is a questionable spelling of a word still considered as slang and not a playable word either. Pulled that off in the second game on the upper middle triple word score for something like 33, also fairly early in the game.