There are numerous examples of significant historical events that, while not well known in their own time, are important in laying the groundwork for later historical turning points years or even decades in the future. Such examples would miss meeting several of your criteria simultaneously.

An example of such an event can be found in the creation of the ARPANET (which was the precursor to the modern internet) in the 1960's (see reference link for more information). Here, it should be remembered: in the 1950's and 1960's, technological limitations meant that the use of computers was largely restricted to large organizations and institutions such as universities. With most Americans having had little if any experience with these machines in general, this earliest work towards computerized networks would serve as one example of an event (or series of events) that would have been of limited impact in its own time, but with lasting implications on technology in the future.

A second example can be found in the voyage of the HMS Beagle from 1831 to 1836, where Charles Darwin made the observations that would shape his work in evolution. After Darwin's return, and his publication of The Voyage of the Beagle in 1839 (to say nothing of The Origin of the Species and The Descent of Man), the voyage of the Beagle would increasingly gain its reputation as a key moment in world history and the history of science, but if we're speaking specifically to the year, 1831, without the benefit of hindsight in knowing what would follow, this would serve as another example.