The One With Joey's Fridge - S6-E19
Corrected entry: In this episode The One With Joey's Fridge, Rachel's hair is just a few inches past her shoulders and in the very next episode, The One with Mac and C. H. E. E. S. E. Rachel's hair has magically grown down to her waist. No one else's hair has changed, and no mention of it in these episodes either. I guess they were just hoping no one would notice it.
Correction: She could have had hair extensions put in, and since we don't see every second of their lives on camera, we could assume her friends already knew about it and did not mention it.
pross79