Since the release of Scala plug-in 0.3.1478 there has been a very noticeable error detecting unused imports. These spurious "Unused import statment" diagnostics come and go.
Here's a screen capture showing one unused import, rrs.util.ERec. This is correctly diagnosed:
(Ignore the errors on the last line for the moment.)
Here's the erroneous "mode." Notice how many more imports are shown in grey:
These incorrect unused diagnostics appear when the project is reopened and disappear when you click around in that file. They reappear when you issue File -> Save All / CTRL-S.
The incorrect errors on the renaming imports of java.lang.System.err.printf and println are a different error that has been present for a very long time.