The union can document over 100 times in the last 5 years. Management says they can prove that on 3 occasions workers were refused the right to accept a personal phone call. In this case the clear and overwhelming pattern is in favor of the union. Three refusals over five years doesn't break up the clear and consistent pattern. On the management side it sometimes is not good enough for a low-level foreman to know, it must be higher management.
Upper management finds out and decides to give everybody warnings for leaving work early.
The union could argue that nobody should get a warning because management did not inform the union that they wanted to change the practice of no discipline for leaving work early on Friday. However since upper management did not know about this practice it would be hard to argue that workers could continue to leave work early every Friday, without discipline. The practice not only has to be known by both parties, it must be accepted by both parties. Often times the fact that a practice occurs frequently over a long period of time indicates that the parties agree to it. A practice that is openly agreed to by both parties, gains past practice status quicker than one that is not openly accepted.