- If you researched all the technologies of the current Era before advancing into next Era, the extra/overflowing Science generated will not be "saved up", until the new technologies of next Era are available.
Technologies that require multiple prerequisite techs can be directly researched if just one prerequisite tech is unlocked. Certain technologies can be "skipped" in this way.
Moreover, each time you research a Technology, all the unresearched techs will get a cumulative 1% cost increase in Science. As a result, late game technologies are going to be more expensive.