This year I am counting the omer with the help of an app. Every evening, just after nightfall I get a reminder to count. It is quite practical actually, not only because of the reminding, but also for keeping track of which day in the omer it is.
At some point I started wondering about the halacha of sending a notification on shabat. Intuitively, probably one might think it is forbidden, as this is an electronic device and basically anything with electricity sets the "forbidden"-flag. But the notifications are automated, so it should be the same as setting an alarm (which you can do) or timers for light (which you can do) before shabat starts. The notification is programmed beforehand by the app developer.
So you might think what does it matter? Anyway, the religious person wouldn’t see the notification, because she certainly turns her phone/tablet/thingy off for shabat. But there are Jews who use their phone on shabat and are still religious (though not orthodox, probably). I know a few who count the omer and use phones on shabat. So if they rely on the app to remind them for the omer and on shabat it dosn’t, and they forget to count… isn’t that near to the category of causing someone to sin? After all they are obligated to count the omer.
I haven’t really looked into this, way too busy at the moment, but I’m curious. As far as I know, my app doesn’t give notifications on shabat. It’s from Chabad, so probably the app developers just don’t think shabat-phone-using-omer-counting-jews exist? Or is there another reason an app shouldn’t send notifications that I have missed?