I have a YA sim who as a teen "snuck" out of the house while grounded. She's STILL performing the "sneaking" animation. I'm beginning to guess that I may well be stuck with it. Has anyone else encountered this?

Your sim is stuck in a particular walk-style, a common occurrence in the game.

Buzzler has a "running disease" mod which also deals with sneaking : http://www.modthesims.info/d/411351

Not the sneaking animation, no, but I've had other things stick on Sims that I could only cause to cease if I saved & restarted the game. Tried that? Tried resetting her?

Hit, CTRL+SHIFT+C
then type, resetsim <sim's first name> <sim's last name>

Buzzler's mod is great for this, but if you don't wish to install a mod, there are two other ways to fix this. You can either send them to a boarding school for 24 sim hours, and then call her back home, OR you can use Edit Town, remove her from the household, and then place her back in right away. The resetsim cheat doesn't always work for this problem.

She isn't a teen anymore. She's aged up to a Young Adult, moved out, gotten married, been reset. . .still, she "sneaks". I've saved and restarted the game probably half a dozen times. But I do thank everyone who has responded. I may have to try the mod. (sigh)

I had this happen to me recently, a teen sim snuck out of the house and got stuck in sneak-walk mode even after her birthday. I sent her to Egypt for her "graduation present" though, and I guess she must have gotten some good seats on the plane because that unstuck her back.

...I thought that was odd, because the last time I flew for 14 hours in coach I'm pretty sure I walked in "sneak mode" for a day.