I'm starting to hear about this phenomena and just wanted to know what are some tell tale signs of a transmission slipping vs any other problem that could also be mistaken for that behavior.
Power is not being fully transmitted from the engine through the transmission to the driven wheels. Engine revolutions do not increase vehicle speed proportionately.
Anything will eventually wear out, but a maintained transmission should not slip.
On a manual transmission, the clutch is likely at fault and the situation will gradually get worse until the vehicle will not move. On an automatic transmission, either internal wet clutches or bands are failing, or the torque converter is failing, or there is insufficient fluid level or pump pressure. This situation will also likely get worse, and can even result in severe damage to the automatic transmission.
Either type of transmission slipping is dangerous in that it could strand you somewhere. A slipping automatic transmission can be ruined and should not be driven.
Either type of transmission can slip if not maintained properly, although for different reasons.
Here's some info I came across for #4 about the potentially dangerous situation a bad transmission poses due to lack of power. I italicized the relevant sentence for emphasis...
Slipping gears: Your transmission is supposed to stay in one gear until you shift (with a manual transmission) or the computer does it for you (with an automatic transmission).
If the transmission is slipping in or out of gear without any effort on your part in a manual, or sliding into neutral in an automatic, you need to get to a mechanic immediately! This is a huge safety issue, because if you need to hit the gas to avoid a dangerous situation, and you have no power to your wheels, the results could be catastrophic. The problem is likely worn or broken gear linkage, so if this happens, don’t waste time – get it fixed. Of all the transmission problems we’ve talked about, most of them won’t get you killed except for this one.
https://www.yourmechanic.com/article/the-10-transmission-problems-to-never-ignore
Gears Slipping This is unnerving at best and potentially dangerous at worst: when you mash the gas pedal to avoid an out-of-control vehicle, the last thing you want is a transmission that doesn't get power to the wheels. No need to scratch your head over whether this is trouble or not: if it happens, you know it's time to have your transmission examined.
