Wow, I thought I was posting first but a few people snuck in there ahead of me
Another thing from the SE4-planning days: Techs with "ors" and maybe even "nots" as well as "ands" in their prerequisites - so maybe you need either Particle Physics or Wave Mechanics to get EM Radar (I seem to recall posting a similar example a few years ago when SE4 was in development
), or you can either research The Light Side Of The Force or The Dark Side Of The Force, but once you research one, you can't get the other!