Skepticism and the Obvious

Once we thought it obvious that the universe was infinite. After all, how else could it be? If the universe is finite, then that means we'll eventually hit a wall traveling in some direction, but whatever is past that wall must still be part of the universe, right?
Relativity turned that idea on its head. Now we are fairly certain that the universe is, in fact, finite in diameter but without any boundaries.

Once we thought that time could not have a beginning or an end, after all what would that even mean? From relativity again we learn that in all likelihood time is finite in a similar manner to space.

Once we thought it was trivial that a thing is either here or not, but quantum mechanics tossed that idea in the trash. When not being observed, a thing can be in a superposition of "here" and "not here" - not merely that we don't know, but that neither intuitive possibility captures the fact of the matter.

What is the lesson from all this?

The more obvious a proposition is, the more thorough you must be in proving its validity - never accept "how else could it be?" as an argument.
If presented with a list of possibilities, always question the exhaustiveness of that list.