To keep the matrices circulating smoothly throughout the machine, it is necessary that oil not be allowed anywhere near the matrix path. If oil is found in the matrix's path (due to careless maintenance or over-lubrication of nearby parts), it combines with dust, forming a gummy substance that is eventually deposited in the magazine by the matrices. The most common result is that the matrix will not be released from the magazine at its usual speed, and almost always results in a letter or two arriving out of sequence in the assembler—a "matrix transposition". When these machines were in heavy use, it was not uncommon for an operator to set type at the rate of over 4,000 ems per hour, with the fastest operators being able to exceed 10,000 ems per hour (approximately 10 to 30 words per minute in today's units) so careful lubrication and regular cleaning were essential to keep these machines operating at their full potential.