I legit burst out laughing at the C solution (https://codegolf.stackexchange.com/a/28889). Definitely got me.

The Haskell one is not that spectacular if you understand that operators are just functions in that language:

    λ> let 2+2=5 in 2+2
    5

This simply defines a new local function that happens to be called "+". It's no different than something like this in JavaScript:

   let Math = { sqrt: (x) => 42 };
   console.log(Math.sqrt(16));

The Ruby versions are a bit underwhelming, this is not 2+2=5 but much more underhanded:

https://youtube.com/watch?v=F3feRCr6S64

(recommend watching it by doing the four "tests" before going to the reveal)

Forth is probably cheating considering it has no restrictions when naming symbols.

  : 4 5 ;
  2 2 + .

Did you try this? This won't work.

What you did will make inputting a 4 yield a 5, but it won't change the behaviour of outputting a 4. (And it will only turn an input 4 into a 5 if your interpreter checks for words before checking for numbers which is not universally the case).

Try : + drop drop 5 ;

this is how javascript works

This reminds me of an SO user that was eventually banned because he (or possibly she, we will never know) had a hobby of writing fairly detailed questions that looked totally innocent on the surface but got strange once you started digging into them. Those that suffered through would eventually realize the whole thing was masterfully crafted to waste your time. There was once an excellent collection of these but I can't find it now and wish for the life of me I could.