Extreme hypothetical here...
We know (SA OC 344:1) that someone who is lost in the middle of nowhere and doesn't know what day it is has special Shabbat rules. They are not allowed to do melacha on any day (except for acts required for survival), but observe every seventh day from the time they realized they don't know as Shabbat, with the special liturgy.
Now, let's mix these two cases together.
A mohel and his wife are lost in the
desert rainforest. He conveniently has all his supplies with him. However, they don't know what day it is. Let's say one day (around noon) (not the day they're observing as Shabbat) his wife gives birth to a baby boy. It's obvious that in this case they should do the mila on the 8th day.
But the birth wasn't at noon. It was in the evening, during bein hashmashot. Now what? We're not sure when the 8th day is, and we're also not sure when Shabbat is. Is safek-8th-day doche safek-Shabbat?
Let's say the above case was on their "Tuesday". What if it happened on their "Friday", so the baby was born just as their "Shabbat" came in?