There are words from which you can remove a "chunk", leaving a new word. Like this:
WISHBONE WI SHBO NE WI SHBO NE WI <poof!> NE WI NE WI NE WINE
There are also words that work the other way, for which inserting a "chunk" produces a new word. For example, you can insert the chunk AUTIFI into the word BEER to make BEAUTIFIER.
A "chunk" is a string of consecutive letters. It must consist of at least two letters (no single-letter chunks). It does not need to be a valid English word.
Now, what if I told you there are words from which you can remove a chunk, then insert a different chunk with different letters, and get the original word again?
What the heck am I talking about?!
There are actually thousands of such examples. I'm just looking for a general description of the pattern that creates this strange phenomenon.
(Too easy? Too hard? Try the counterpart subtraction paradox.)
Here is the specific example which motivated this post:
Start with the word QUARTERBACK and remove the chunk RTERBA to obtain QUACK. Now, take the new chunk ARTERB (which is obviously different from the chunk that was removed) and insert it into the word QUACK to obtain the original word QUARTERBACK again.
Perfectly identical, but not exactly the same! It's a little bit of a shell game, and if you followed it with sharp eyes, you might have noticed that it was the first "A" in QUARTERBACK which was retained in QUACK, but then it became the second "A" when QUARTERBACK was restored.
The two "A" are like sentinels which stand on either side of a middle string of letters. The chunk which is removed must contain the middle string of letters as well as one of the sentinels. The chunk which is inserted must contain the middle string of letters as well as the other sentinel.
@GarethMcCaughan did a good job below of exploring whether the sentinels can be more than a single letter. They can be!