Why does my Android phone think it was in Japan? According to Google location history, I was in Tokyo in February. It could be that I had so much saké I'm forgetting something important, but as far as I know I've never been to Japan.

Other questions on inaccurate location history seem mostly related to minor inaccuracies because of phone connecting to nearby WiFi points.  

But what technical issue could cause the location to be off by ~6000 miles?
One reason could be that that location was determined from a WiFi access point, which had been previously moved from Japan to somewhere near you.

Google registers the location of each WiFi access point Android devices are connected to (if allowed in settings) and uses these to obtain a location when GPS is slow or unavailable. Unfortunately it takes some time to update the location if the access point is moved, so this kind of error can happen.