When Charlotte bends down to examine the tire tracks on the power plant property, there's a big strand of hair hanging in front of her left eye. The camera cuts to the tire tracks, then immediately back to her face and her hair is back in place.
When Jonas went to school in 1986, he asks Regina what day it is. She answered Nov. 9th. But Nov. 9th in 1986 was a Sunday and there are no schools in Germany that would have had regular classes on a Sunday.
When Egon meets Jonas (around 00:12:30), who is walking along the road in the rain, he asks him if he isn't "supposed to be in school right now". Since it's a Sunday, schools in Germany would have been closed and Egon would have known that.
At least, it's a consistent mistake - the school seems to also have been open and full of students earlier too, when Jonas asked Regina about today's date.
When the elder Helge is walking into the cave, he moves under the police tape. When Ulrich follows behind him, it looks like the tape is gone, but it's lying on the ground and not very visible due to the darkness.
At 7:14 of this episode Egon Tiedemann looks at a picture of Mads and Ulrich Nielsen which lies on top of the witness report about the rape accusation against Ulrich by the teen-aged Hannah Kahnwald; Kahnwald is however the name she took on when she married Michael Kahnwald. At the time her name should have been Hannah Krueger.
When Ulrich Nielsen walks into the pitch black cave after Helge Doppler, the cave is quite comfortably lit on the inside, even before Doppler turns on the lamp.
The old cop Egon in 1986 seems puzzled by Jonas' 2019 ear buds. But small ear bud earphones, albeit usually a single monotrack, had been common with transistor radios for many years prior to 1986.
When Charlotte hangs up the phone on her husband, a busy tone sounds from his mobile phone, but there are no busy tones on mobile phones after hanging up.