At least once a day, IDEA decides that my Scala files should be using 4 space tabs instead of the 2 space tabs that I have configured for all Scala files. This only affects new lines that I add to the file, not existing lines, but obviously it throws off the formatting of the file. And of course I can't fix it using the backspace key, since the IDE is "smart" enough to know that if I'm backspacing at the beginning of a line, that must mean I want to delete a line, so it joins the current line with the previous line. The only way to fix the spacing is to select and delete the extra two space characters (very tedious).
I have reported this (https://youtrack.jetbrains.com/issue/SCL-8664), except that the issue is still "waiting for reply" even though I have done my best to provide the requested information.