Mistakes that can't be repeated (5,6)

'can't be repeated' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'mistakes' is the wordplay.
I cannot quite understand how this works, but
'mistakes' could be 'errors' ('error' can be a synonym of 'mistake') and 'errors' is present in the answer.
The remaining letters 'fatal' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'that' is the link.

Can you help me to learn more?