GPS is not expected to work properly underground as it requires a clear signal from the satellites; also, signals from different satellites are needed to triangulate a precise position (such the ones shown in the movie), which is not plausible underground.
A grizzly bear weights about 250kg. When Marc gets flattened by the bear after he had speared it, he should at least report some broken ribs.
Skyscrapers' windows' panes cannot be broken by just throwing a chair at them.
A single bullet from a small gun like Marc uses in the sewers is not powerful enough even to just pierce a gas can; even if it worked, there's no reason for the can to explode.
While Enrique collecting seeds for the future might seem like a good idea, cooked (from microwave popcorn) or baked (in bread) seeds would not germinate.