That one Stanton actually investigated. Members of the 20th Maine claimed to have seen a tall man wearing a tricorn hat riding a white horse (which would be Washington's white war horse Nelson) who directed them to Little Round Top. Chamberlain told Stanton, "We know not what mystic power may be possessed by those who are now bivouacking with the dead. I only know the effect, but I dare not explain or deny the cause." So...basically he didn't know what his men had seen but however it happened they were where they needed to be that day at Gettysburg!