Quick fix: Don't panic — the stitch is almost certainly still there, held in the waste yarn. Use a crochet hook to pull the loop back up and onto your needle before removing more waste yarn.
What you are seeing
You're removing the waste yarn from a provisional cast on and a live stitch has slipped off the needle or fallen down into the fabric. You now have one fewer stitch than you should, and a small ladder or gap where it should be.
Why it happens
- Removing waste yarn too quickly before all live stitches are secured on the needle
- Not placing live stitches immediately onto a needle as the waste yarn is removed
- Slippery yarn making it hard to grip live loops
- Stitch placed back on needle with wrong orientation — loop slips through
Fix it now
- Stop removing waste yarn immediately. The dropped stitch is likely still attached in the row below.
- Look for the loose loop on the fabric near the gap. Insert a crochet hook through it from front to back.
- With the hook, pick up the horizontal bar above the dropped loop (this is the live stitch) and pull it through to re-create the loop.
- Place the recovered loop onto your needle, checking that the right leg sits at the front of the needle.
- Continue removing waste yarn slowly, placing each live stitch directly onto the needle as it's freed — don't let stitches pile up uncontrolled.
Prevent it next time
- Use a needle the same size or slightly smaller than your working needle to catch live stitches as you remove waste yarn
- Remove waste yarn 2–3 stitches at a time, not the whole row at once
- For slippery yarn, use a needle with good grip (wood or bamboo)