It sounds like it might be transmission slip. That would cause the engine to rev up as the engine would no longer be engaged with the drive train (similar to being in neutral). There are additives on the market that claim to stop slipping, but these are only temporary fixes. A slipping transmission can only be fixed truly by rebuilding or replacing the trans.

Clutch slipping

Does it have to give warning signs before it fails?

trans is slipping. start with checking the fluid level.

no its been driving fine

Driving on freeway revs raised and car slowed down?

2005 ford torus auto was driving on freeway and started to rev but slow down no warning sign that tranny was going bad Fluid lvl looks fine i was litarly driving and the revs went up and i slowed down no clunk no sound no warning nothing

auto shifter slides right between all the gears prnd12 with no resistance