In a well-known investigation of session-to-session variability, McGonigle et al. published a data set in which a single subject participated in 99 imaging sessions (33 repeats of simple finger tapping, checkerboard viewing, or counting). These two papers present different views on how the results might be interpreted.
McGonigle DJ, Howseman AM, Athwal BS, Friston KJ, Frackowiak RSJ, Holmes AP (2000) Variability in fMRI: An examination of intersession differences. NeuroImage 11:708-734.
Smith SM, Beckmann CF, Ramnani N, Woolrich MW, Bannister PR, Jenkinson M, Matthews PM, McGonigle DJ (2005) Variability in fMRI: A re-examination of inter-session differences. Hum Brain Mapp 24:248-257.