Both experts swear this is extremely unlikely. Toxic shock syndrome (TSS) is a very rare but life-threatening complication that can happen as a result of a bacterial infection, and it was associated with the use of super-absorbant tampons in the past. When it was most prevalent in 1980, the rate was only 6 to 12 per 100,000 menstruating women (per the CDC). By 1986, that rate dropped to 1 in 100,000. And, obviously, tampons have changed even more in the last 30 years. That said, it can happen, so if you have a super-high fever, you're nauseous, and your skin is flaking off (the major symptoms), call your doctor ASAP, says Minkin.
But really, you shouldn't be too worried. It's safe to leave a tampon in overnight, says Minkin. Just always use the least absorbant tampon you can get away with, and don't forget about it. Seriously, both experts have had several patients come in with tampons that were left in and forgotten — and they were fine (though smelly).