Tell someone or document it before you close it up.
I already cut the duct, opening, or metal wrong. What now?
Stop before making the short/wrong cut worse. Do not hide it. Mark the piece, take a photo, measure what is short or wrong, and ask the foreman whether to repair, remake, reorder, or document the issue.
Stop, document, measure the actual mistake, and ask before changing it more.
Do not keep trimming until it “sort of fits.” Do not hide the mistake with tape, mastic, insulation, ceiling tile, or paint.
Check
What was supposed to be cut?
What got cut instead?
Can the piece still be used safely and cleanly?
Does this affect access, seal, support, rated work, airflow, or finish appearance?
How short is it compared with the drawing, fitting, or connection requirement?
Steps
Stop cutting and move sharp edges out of the way.
Take one wide photo and one close photo with a tape measure if it helps.
Measure the actual cut versus the drawing, fitting, opening, or layout mark.
Mark the piece so nobody installs it by accident.
Ask before trimming more, forcing fit-up, sealing, insulating, or covering it.
If the duct or piece is too short, do not stretch, force, over-seal, or hide the gap.
Say this to your foreman
I cut this duct/piece too short or wrong at [location]. I stopped before changing it more. Here is the photo and measurement. Do you want this repaired, remade, reordered, or can it be adjusted another way?