SNCF's old-style orange "composteurs" (ticket validating machines). They used to punch a hole (or rather half a hole) in the edge of your ticket and stamp it on the back (or on the front if you inserted your ticket face down) with a series of numbers containing some secret information about when and where it was stamped, including, I believe, the day of the year (e.g. 001 for 1st January, 365 for 31st December - so I don't know if they could be programmed to do 366 if it was a leap year) and the station reference number.
If you had a standard SNCF ticket, and you inserted it face up, you had to be careful not to insert it right by the magnetic strip on the back, or it would get stamped on the strip and the train conductor might think you hadn't validated it and fine you.
As well as standard SNCF tickets, you could also put any old piece of paper in them. When travelling in France using British Rail International tickets issued as booklets, my parents and I always used to validate the relevant coupon to be on the safe side, although we were never sure whether tickets issued by carriers other than SNCF still had to be validated.
There are still (or were until recently) a few of the old style validators in use at the Gare du Nord, but they are now pale grey rather than orange, and they have mostly been replaced with new style yellow validators which only take standard format SNCF tickets and stamp the actual date, time and station on it. Some of the new style machines work no matter which way up and which way round you insert your ticket, and some of them only work if you insert it at the left-hand end, face down.