Sager highlights a study that had partisans of the Arab-Israel debate watch a news report on the subject:

[T]hese groups watched the same news and came to opposite conclusions as to which way it was biased. And each side thought it was biased against their side.

PsyBlog explains the phenomenon:

There were two mechanisms at work here:

  1. The truth is black and white: partisans generally thought that the truth about the Arab-Israeli debate was black and white. Any hint of shades of grey in the news reports was interpreted by partisans as bias towards the other side. In other words: any balanced report will seem biased to partisan viewers.
  2. The news report was too grey: as well as thinking the Arab-Israeli issue was either black or white, partisans also perceived that the specific news report they watched was too grey.