In the sixth series where Chandler accidently picks up the telephone at Joey's house, it is new timings for auditions for the sitcom Mac and c.h.e.e.s.e., behind Chandler on their notice board on their door, there is a drawing of an alien of some-sort, yet when Chandler goes to write the message for Joey, it mysteriously disappears!
I just saw this ep. last night and lipi is correct, I caught it also. He turned around and immediately started to write and there was nothing on it. I didn't look closely enough to recognize what was on there but I do remember seeing something and then it was gone.
The message board is called an etch-an-sketch. You can easily erase it by sliding the handle at the bottom of the board. This could be done quite easily. But nevertheless it did seem that he did it quite quickly.
The thing Chandler wrote on wasn't called Etch-A-Sketch, it was a Magna-Doodle - you write on it with a little magnetic pen. Etch-A-Sketch draws pictures by turning knobs on the bottom and erase by shaking it.
Actually, the thing on the door is called a "Magna-Doodle", not an "Etch-a-sketch". Though similar in many ways, the "Etch-a-sketch" erases with a shaking motion, not like the swipe move Chandler made with the "Magna-Doodle".