The short story: I cursed a lot and ripped out a couple inches of knitting.
The slightly longer story: I had (for once) inserted a lifeline in my knitting. BUT had to rip out to a point before I had threaded the waste yarn. 🙁
WHAT IS A LIFELINE ANYWAY?
A lifeline is a piece of waste yarn that you thread through a row of your knitting, so that if you have to rip back, or if you drop a stitch, the lifeline will catch your stitches.
Lifelines prevent your stitches from unraveling back to your cast on.
CHOOSING YARN FOR A LIFELINE
The best lifelines are ones that are skinny, smooth, and highly visible.
I usually use whatever lace weight or fingering weight yarn I have hanging around, but crochet cotton, or dental floss (the non sticky kind) also make good lifelines.
Don’t choose a color that blends right into your fabric, it just creates a headache.
High contrast lifelines are a very good thing.
THREADING THE LIFELINE
Grab a tapestry needle, and thread your waste yarn.
Then slip the tapestry needle with the waste yarn through each stitch on your needle. Remember to avoid any stitch markers. You do NOT want to thread your stitch markers onto your waste yarn.
Another way to thread a lifeline is to anchor the waste yarn to the needle and allow the needle to draw the lifeline through the stitches as you knit a row. Here’s a detailed tutorial from Knitting Yards.
BONUS PRO TIP
Lifelines help a lot more if you put them in before you need them.