If you think about it, each month you 'miss' 7 pills when you have your week off between packs. You have breakthrough bleeding and you are still protected from pregnancy even though you've had quite a big gap between pills.
The risk of missing first 7 or last 7 is because it effectively extends the pill free period beyond what is safe to prevent pregnancy. If you miss one of the middle 7, enough hormones are in your system that there is no dip in protection.
As another example, if you missed the last pill in your packet, an easy way to deal with it is to simply treat that as the first day of your seven days off and start the new pack after a seven day gap. You'd have a new start day for your new packet, but because you'd just had the standard 7 pill free days, you'd be fine.
It is really just convention to mimic a natural cycle that we take pills at 21 then 7 days off. For example, they could easily be manufactured as 35 pills and then 7 days off.
I've only heard of negative results in late pregnancy. The hcg drops as the placenta takes over, so you are talking quite far along. I think by the time there was any risk of a false negative there would be quite a number of other external signs of the pregnancy IYSWIM.