At least twice in recent days, I’ve seen people using “in dem” and “indem” incorrectly (I think one each of the two possible directions of mix-up).
It should be simple, really: the two-word one is the more literal “in which”; the one-word one is the fossilised “by” indicating the means:
“Er öffnete den Kofferraum, in dem er das Schloss transportiert hatte.”
“Er öffnete den Kofferraum, indem er das Schloss knackte.”
Annoying, especially because I hadn’t seen this particularly misuse before, and since the two are used rather differently syntactically, my mind was completely garden-pathed.
(By comparison, I think I’m less confused by people using the wrong spelling from the set “they’re, their, there”, because I’ve seen those errors too often.)