What is likely the most well known/infamous example of this happening is Hitler, who was a foot soldier in WW1 and the leader of the Third Reich in WW2. Although he wasnt strictly a general, and being the leader of the state doesn’t necessarily mean you are heavily involved in military decisions, Hitler was.

There are a number of good threads on /r/askhistorians that do a better job of expanding on Hitler’s tactical abilities than I can. In particular I would point you to this post about Hitler’s military acumen. The very short answer from that thread is basically that he did not overcome his deficiencies in tactical knowledge.