Unfortunately, this can be very dangerous and this is what can happen if you leave it in for too long.
Toxic Shock Syndrome is oh so real, and VERY dangerous. Potentially fatal. TSS is caused by a build up of bacteria from leaving the tampon in for a prolonged period of time. Symptoms of TSS can include high fever, low blood pressure and skin that looks like it has been burnt by boiling water. Get to a doctor as soon as possible if ever experiencing these!!
Tampons are created to soak up all your period, so if they're left in for too long, they're going absorb everything and dry out your vagina walls, which could lead to a lot of discomfort.
If tampons dry your vagina too much, switch to pads for a while until you recover your moisture.
If you have a heavy flow and your tampon is full, then naturally some period blood may leak through. Especially if you've had your tampon has been in for more than 12 hours.
Periods smell. But sometimes, when a tampon is left in for too long, you will smell it. Ick!
It's the blood that smells, and when it starts to interact with the bacteria in your body, it will develop a stronger scent. The longer the tampon is in, the more pungent the smell will become.
It's completely dangerous to leave a tampon in, but it's also possible that absolutely nothing will happen. However, it is HIGHLY recommended that you change your tampon every 4-8 hours to avoid the above.