The most common and most likely answer to this crossword clue is the 7 plus 10 letter word SLIPÂ OF THEÂ TONGUE
I've seen this crossword clue in The Guardian
S
L
I
P
Â
O
F
T
H
E
Â
T
O
N
G
U
E
Buddy explains!

The straight part
We know that "unfortunate mistake" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unfortunate mistake = slip of the tongue.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.