Ok, so this is probably something you all know but I’ll share anyways.
When you’re knitting (especially sleeves) and there is a gradual increase in width, this is more than likely done using a M1 instruction (make one).
To do this without leaving a hole:
pick up loop which lies before next stitch (from the front) and place on the left-hand needle (make sure you bring the yarn that’s on the right side of the needle over the left hand needle.
knit into the BACK of this loop, this will twist the loop and prevent a hole from appearing in your work.