In May 323, shortly before Alexander the Great left Babylon to conquer Arabia, a strange incident happened: a man of very humble origins sat down on Alexander's throne. The Greeks and Macedonians regarded this as a very evil omen.
However, the incident can easily be explained. The ancient Assyrians and Babylonians believed that if an evil omen threatened the king, somebody else - usually a prisoner or another person of low origins - was to sit on his throne. Evil would hurt him, and the real king would remain safe. The Persians seem to have had a similar custom: the Greek researcher Herodotus of Halicarnassus tells us that when Xerxes had terrible dreams, he had himself substituted by his uncle Artabanus.note
So, it seems that the Babylonian astrologers had appointed a substitute king, that the eunuchs knew what was going on, but Alexander did not. If this interpretation is correct, the astrologers were the most loyal subjects the king could wish.
Arrian of Nicomedia describes the incident in his Anabasis, sections 7.24.1-3; he has his information from Aristobulus.
They are given here in the translation by Aubrey de Sélincourt.
The substitute king
[7.24.1] Alexander's end was now rapidly approaching. Another portent of what was so soon to come is mentioned by Aristobulus.
While the King was engaged in incorporating in the various Macedonian units the troops which had come from Persia with Peucestas and from the coast with Philoxenus and Menander, he happened to feel thirsty, and getting up from where he was sitting moved away and left the royal throne empty.
[7.24.2] On either side of the throne stood couches with silver feet, on which his attendants had been sitting, but they had got up and gone away with the King, and only the guard of eunuchs was left standing round the throne.
Now some fellow or other - some say a prisoner under open arrest - seeing the throne and the couches unoccupied, made his way up through the eunuchs and sat down on the throne.
[7.24.3] The eunuchs, according to some Persian custom, did not turn him off, but began to tear their clothes and beat their breasts and faces as if something dreadful had happened.
Alexander was at once told, and ordered the man to be put to the torture in an endeavor to find out if what he had done was part of a prearranged plot. However, all they could get out of him was, that he acted as he did merely upon impulse. This served to strengthen the seers' forebodings of disaster.