I watched a program on Discovery or Animal PLanet a while back which featured frogs and other amphibians. One image that has stayed with me is that of a very large bullfrog stuffing various other creatures into its mouth–even small vertebrates such as mice! Some of its prey were about as large as the frog itself–it had not bitten its prey into pieces, so it was basically intact and still struggling, despite being forced into the gullet. It occurred to me that once inside the mouth, an animal could actually begin to bite and counterattack and perhaps even overcome its predator! Has there ever been a recorded instance of this happening?
I don’t know if it’s been observed, but i don’t think it would happen very often for a couple reasons:
First, what is there to bite? The slippery inside concave surface of a throat or esophagus that wants to slip away being somewhat elastic… I’m thinking of wraping your head with a lubricated trampoline tarp and then trying to bite it; a lot harder than biting something you can actually get your jaws around.
Second is that once you’ve been jammed into some animal’s thoat you can no longer breath. So IF you happened to take a deep breath before getting half swallowed you’d probably only have under a minute before you’d be out especially considering all the struggling and stress you’d be under. Not to mention all the trauma your predator is causing you by crushing the rest of your body in his jaws (even if you aren’t getting sliced up). These conditions probably aren’t very condusive to level-headed problem solving in the few short moments the prey would have left.
Wow…I’m flashing Ray Milland here…
Most animals have a strong bite but have little strength when it comes to opening their jaws. Biting/chewing/bonecracking build muscle. Just opening jows don’t.
Witness the crocodile/alligator teasers who hold ther snouts closed with their hands. They need an assistant to tie a rope or let go very quickly and dodge the beast’s jaws and tail lash.
Same for a frog catching and eating a small or not so small animal. Just get a grip on it and it’'s gone down the hatch.
Same for snakes. The snake can unhinge it’s jaws and swallow a mouse, rat, or large snakes can swallow a pig. It take a week or so to digest such a large meal.
As mmmiiikkkeee noted, it’s possible but usually not very likely. Sometimes stinging and nastily toxic critters will be spat out ( a classic early experiment on mimicry was conducted using stinging bumblebees and tasty bumblebee look-alikes and the frogs would quickly get gun-shy of grabbing bumblebees ), but even poison isn’t always a defense. Witness the grand champion of them all - the African Bullfrog ( Pyxicephalus adspersus ) who swallowed seventeen hatchling cobras 
( Rinkhals in this case ). Granted that species does have a few big teeth on the lower jaw, unlike most frogs ( who usually just have some small ones on the upper jaw for grabbing and holding ).
- Tamerlane
While escape and survival are both unlikely, it’s possible for prey to get the last laugh. I think it was Alice Cooper whose beloved snake died after eating a live mouse that scratched it, causing an infection.
As a frog owner I have been advised to crush the jaws of Zophobos (v. large meal worms, they look like Dune worms) if feeding them to my White as they are capable of putting in a few good bites after being swallowed.