you mean you picked the dropped stitch back up or you mean you made a stitch in its place a few rows later?
in the case of you picked the dropped stitch back up...no worries... The "hole" you see is just the fact that you stretched the stitch...normaly when you block it will fix it. But i dont EVER block my blankets. so what i would do is pull the neighboring stitches just a bit and it should fix it.
in the case of you made a stitch in its place...you now have a stitch loose somewhere down in your knitting and you will have more holes than you can shake a stick at...you need to go find it and catch it now.
hope this helps