This has happened to me too.
Part of the problem is that it's hard to grab the filter holder.
So find a wire coat hanger (or something similar) and straighten it.
Bend a hook into one end of it.
Lower that into the filter holder and hook the bent end around the bottom of the filter holder.
You can then pull the coat hanger up, maybe with jerking motions, to pull the filter holder out of the pitcher.
In the picture below, the red thing is the wire coat hanger.
lifehacks.stackexchange.com/users/4198/BrettFromLA
I've made this same mistake many times myself and it always frees-up if you just wiggle it around a little.
The material of the jug and the funnel are both very strong so don't be afraid to apply a little force to them.
lifehacks.stackexchange.com/users/1965/Lefty
Maybe wash them with soap and then pull them apart as @Lefty said, because the soap will act as a lubricant.
lifehacks.stackexchange.com/users/24450/Trajan Espelien
Dip the bottom part into very hot water for some 15 seconds.
The heat will cause "slight stretching" of the outer piece.
Then wiggle the stuck part out.
lifehacks.stackexchange.com/users/24761/Suzana
This happened to me.
I took the filter out and then put a tea towel down on the counter and smacked the jug against it (so the open bit was hitting the counter)...
it ended up falling out.
lifehacks.stackexchange.com/users/28380/Leanne
Hot water worked for me - jug and cartridge holder already sitting in hot washing up water, because I was about to wash it when I realised it was stuck, so probably had longer than 15 seconds.
In as long as it had taken me to find and read the answers here, the top slipped out easily.
I did fill the jug completely to the top with hot water; after immersing it in the sink, I topped up to overflowing by ladling it in with a mug.
Try this solution first - lot less hassle and less danger of damage
/scratches to self or jug than the coat hanger option!
lifehacks.stackexchange.com/users/29798/user29798
When I clean Brita water filter jug, I accidentally placed the filter holder in the wrong orientation.  As a result, the filter holder is stuck in the water filter jug.  I tried very hard to remove it, but I did not manage to do it.  Is it possible to take the filter holder out of the jug?If yes, please advise.  I have attached an image for your reference.    Thank you.
How to remove filter holder out of Brita water filter jug?
lifehacks.stackexchange.com
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lifehacks.stackexchange.com/questions/18723
lifehacks.stackexchange.com/users/24736/hunterex
There are a few ways to remove a stuck filter from a Brita water filter jug. You could use a wire coat hanger as a hook to pull the filter hold out, or you could lay a tea towel on the counter surface and bang the jug on it until the filter holder falls out. Other possible techniques involve using soap as a lubricant or simply wiggling it around a little. The options are to use a coat hanger, wiggle it around, wah first with soap, dip into hot water for 15 second, smack against something hard wrapped in a towel or leave to soak a while first in warm soapy water.
There are a few ways to remove a stuck filter from a Brita water filter jug. You could use a wire coat hanger as a hook to pull the filter hold out, or you could lay a tea towel on the counter surface and bang the jug on it until the filter holder falls out. Other possible techniques involve using soap as a lubricant or simply wiggling it around a little. There are several ways of removing it. You could use a wire coat hanger and straighten it to hook out, simply wiggle it or wash with soap and pull apart. Another way is to dip the bottom into hot water for 15 seconds.
This often frees up if you wiggle it a little, and soap might also help as a lubricant. Alternatively, you might try tapping the jug on the counter, protected with a tea towel. There are many potential ways to loosen the filter holder. You could try simply wiggling it back and forth or banging the jug against a hard surface (covered with a tea towel). Alternatively, you could try washing it in soapy water to allow the soap to act as a lubricant.
Use a coat hanger to prise the filter holder out. Wiggle it around. Wash with soap and then pull. Dip the bottom part into very hot water for 15 seconds. Place a towel on the counter and smack the jug against it. Leave it standing for a while in hot soapy water.
Use a wire coat hanger and straighten it to hook out. Wiggle it. Wash with soap and pull apart. Dip the bottom into hot water for 15 seconds.
Wiggling the holder may loosen it up. Washing using soapy water will act as a lubricant and potentially free the holder. Banging the jug against a surface covered with a tea towel may loosen the filter holder.