Back in the early days of the F/SN novel when Shirou still had many routes/girls to go after xD (rip Medusa, Medea, proto Shielder, and last but not least... rip Illya's route...)
Medea : Are you my master? *Proceeds to die for Lancer*
If you look closely, Shirou has no dried blood stains on his uniform, meaning he may not have summoned Medea after having been stabbed by a spandex bodysuit wearing Irish hero.
Though it still seems to be an accidental summoning.