I'm often asked to explain the doctrine of estoppel and how it applies in the labour relations context.  I found this nice quote in University Health Network v. Ontario Nurses Association issued two weeks ago:

54. Both parties referred to the recent Supreme Court of Canada’s recent decision in Nor-    Man  Regional Health Authority Inc. v. Manitoba Association of Health Care Professionals, 2011 SCC 59 (CanLII), 2011 SCC 59 in which the Supreme Court upheld an arbitrator’s award that had adapted and applied the equitable doctrine of estoppel. The Supreme Court found that labour arbitrators are not legally bound to apply equitable and common law principles, including estoppel, in the same manner as courts of law. Labour arbitrators have a different mission, informed by the particular context of labour relations.

55. The Supreme Court cited Paul C. Weiler’s, (then) Chairman of the British Columbia Labour Relations Board, decision in Re Corporation of the City of Penticton and Canadian Union of Public Employees, Local 608, (1978), 18 L.A.C. (2d) 307 (B.C.L.R.B.), at p. 320 regarding why the doctrine of estoppel must be applied differently in a grievance arbitration than in a court of law:

    " ...a collective bargaining relationship is quite a different animal. The union and the   employer deal with each other for years and years through successive agreements and renewals. They must deal with a wide variety of problems arising on a day-to day basis across the entire spectrum of employment conditions in the workplace, and often under quite general and ambiguous contract language. By and large, it is the employer which takes the initiative in making operational decisions within the framework of the collective agreement. If the union leadership does not like certain management actions, then it will object to them and will carry a grievance forward about the matter. The other side of that coin is that if management does take action, and the union officials are fully aware of it, and no objection is forthcoming, then the only reasonable inference the employer can draw is that its position is acceptable. Suppose the employer commits itself on that assumption. But the union later on takes a second look and feels that it might have a good argument under the collective agreement, and the union now asks the arbitrator to enforce its strict legal rights for events that have already occurred. It is apparent on it face that it would be inequitable and unfair to permit such a sudden reversal to the detriment of the other side…."