Underarm hole in knit sweater, how to prevent it?

I myself have only completed 1 sweater and it is a 3 color baby sweater with stripes and the sleeves were formed from picking up stitches at the bind off edge of the shoulder. The pattern used to be on Lionbrand but is discontinued now(I honestly have searched for it making sure I have typed “stripe” because that is part of the pattern’s name and could not find it)

Anyway, after picking up the number of stitches the pattern called for and knitting the sleeve, I noticed that there was an eyelet size hole at the underarm, Luckily I can just sew it up but how can I prevent it from happening?

Also with the fair isle sweater that I have put on hold for now, the sleeves are knit differently. With that pattern I cast on separately for the sleeves, knit according to the pattern, then join the sleeves to the body of the sweater at x round(I actually don’t remember which round it is) and continue knitting as if it were the body of the sweater. Will I get a similar size hole with this join in the middle of the round(both sweaters are knit bottom up)?

I usually pick up two extra sts on each side of the underarm bind off. On the next row, k2tog or ssk to decrease those sts. That’ll usually close up any hole. It also helps not to pick up sts too close to the edge especially at the underarm.

With sweaters with a yoke which sounds like what you’re describing, there are usually sts bound off for the underarm on both the sleeve and body. Yes, you can get a hole here too. Yarn tails can be helpful here for closing the hole. It seems that you could also close any hole when you graft or seam the underarm.

Yep, what @salmonmac said. I just pick up a few extra stitches and then k2tog them away on the next row.