Stochastic modeling of citation slips
  We present empirical data on frequency and pattern of misprints in citations to twelve high-profile papers. We find that the distribution of misprints, ranked by frequency of their repetition, follows Zipf's law. We propose a stochastic model of citation process, which explains these findings, and leads to the conclusion that 70-90% of scientific citations are copied from the lists of references used in other papers. 
M.V. Simkin and V.P. Roychowdhury