The appearance of the dead king in full armor certainly seems to foreshadow some disruption in the court of Denmark. Why would the king's ghost need to return at all if there were not some wrong done him during his life, and why would he come dressed for war if he only had some insignificant thing left undone or unsaid? The presence of his ghost, and also the armor he wears, seems to signify some menacing thing, a terrible sin or some coming doom.
In addition, Marcellus's famous line, "Something is rotten in the state of Denmark," furthers this idea and adds to the foreshadowing (1.4.100). He says this when he and Horatio take Hamlet to see the ghost of the dead king, Hamlet's father, and this line, combined with Hamlet's earlier assertion—that Denmark is now an "unweeded garden / That grows to seed. Things rank and gross in nature / Possess it merely"—seems especially foreboding (1.2.139–141).
This motif of rot or decay runs throughout the text, and this reference even seems to allude to the Garden of Eden, the home of Adam and Eve in the Book of Genesis in the Old Testament of the Bible. After they disobey God, they lose their innocence and are kicked out of the garden forever. Hamlet seems to think of Denmark as a place where innocence has been lost and where God's law has been broken as a result of his mother's hasty remarriage to her brother-in-law, his uncle. Adam and Eve remain barred from Paradise forever, and so this forebodes an unpleasant future for Denmark.