My personal imposter syndrome feelings are almost always related to either a) not knowing the "replacement" to an existing technology I am already familiar with (in almost all cases it's two steps forward one step back anyway), or b) not using a technology for a somewhat extended period of time, then coming back to it and realising that the landscape and ecosystem has completely shifted from underneath me.
The latter has hit me most hard lately in iOS development with the move to Swift (and then v2, v3), deprecation of NS*, autolayouts, APNS' http2 etc etc, and in JavaScript development.
We have far too many technologies that have established niches that all accomplish largely the same goals, but aren't transferrable. These days, boosting the toolchain for these technologies is a big feature too, which increases the differences between ecosystems. They require sometimes hundreds of hours of learning to become competent in, even for an expert in another similar technology.
It's some sort of crazy arms race that we don't seem to want to stop...