The only time I've ever ignored it was travelling as SLF from LHR to AKL. The captain put the sign on due to some minor turbulence and obviously forgot to turn it off again.
After 2 HOURS of having a baby with a pooey nappy sitting on my lap I thought that enough was enough and got up to change the nappy. I popped my head into the galley to ask if it was alright and was told that this captain often forgot to turn it off!