I agree.... very puzzling. I have never seen leaves turn brown like this JUST on the margins. I seriously doubt this is from a water problem- either too much, or too little. Do you fertilize? If so, with what and how often? Is the rest of the leaf also turning brown and dying?

I agree with Michaela, it is odd. If they were underwatered, which take some time, the entire leaf would be soft and wrinkled.
Note that variegated tissue is more sensitive than the green normal tissues, so if the plant maybe got too cold, or suddenly exposed to much more sun than it had previously, it would be possible for the yellow edge to get burned but the central green part of the leaf to be fine.

Hi,

first of all, I do not think this is a serious problem. The plant as a whole looks healthy.

@paxflower: where do you keep the plant? My idea is that it has a lack of light.

I am not the expert that you asked for but I have been growing a lot of San. for the past 30 years. Fist of all I would be suspicous of your watering practice. I do not water my San. unless I see slight wrinkling of the leaves and I put a chop stick all the way down into the soil, just to check and see how dry it is if completly dry I wait three more days and water from the botton, it can and is caused by many different reasons, dry root ball, it has turned to mush I would take it out of the soil, and check for root diseases, fungus, mealies, no water reaching the root ball, I do not water my San. as soon as the weather starts to change, and here it is Oct 1, they do not like drafts, quick changes of climate. Fertililzer or too much water this time of the year, I grow mine rather dry, and not lush, the plant that you have can handle more water than most, as it has soft green tips which tells me that. What surprises me is that rather new leaves are turning brown, this can mean it is in diseased soil, or soil that is not well draining and keeps the plants to moist most of the time. Or to close to a heater. Is it in Peat Moss? That could be a problem, what the suppliers call cactus mix, I use for my African Violets. I put in additional pumice 50% to the cactus mix. how warm are you keeping it at night? It does not like certain kinds of heat it is an African plant, and will need to be allowed to go dorment. I am telling you from 18 years of experience working as a volunteer at the HBG and have been trained to take care of this species. Dry heat it will be very unhappy with, and actually will need more humidity that you can provive it with by just setting some water in a pot along side of the plant. Do not let it sit in water.
Do not fertillizer intil your weather is stable. Remove all Peat Moss, Make sure the soil is Fast draining, my friend Pirate Girl is from New York be sure to ask her how she grows them if you come from any State East of the Rockies. Make sure that they have air circulation but not diretly on the plant, and give it some light close to a Southern Window, or 6 feet away from a Western window. it need good indirect light. Sorry for writing this book, but I just want to make sure you have enough information to guide you. If it's in a clay pot, it could be drying out too fast. Does your pot have a hole in the bottom? I don't know your cultural practice. I don't know what condition that your are growing your plant, do not give it little sips of water except if is wrinkled like an old lady, they do drop leaves, like trees this time of the year, and like we drop hair. I hope I have been of some help, Norma