There was this one time where I was talking with one of my friends from school while walking down the school corridor and at the other end of the corridor on a wall was a poster that was quite literally bright red and had blue writing on it saying "GO LUCID NOW!". I pointed it out to my friend and laughed at such a funny coincidence; I had wanted to become lucid and here was a poster telling me to go lucid!

I just laughed it off and nearly headbutted a wall when I woke up for being so stupid. I've never had a more obvious dream sign.