In Fred Meyer Stores, Inc.,[1] the National Labor Relations Board (NLRB) found that the retail store and the union had a "past practice" of allowing one or two union representatives to visit employees on the sales floor for "a minute or two."  However, because the NLRB found there was NO "past practice" regarding how many union agents could visit the store, it ruled that the employer could not lawfully react in response to the union blitzing the sales floor with eight union agents at one time.  Because the employer had agreed to a visitation clause in the collective bargaining agreement, the NLRB ruled, the employer could not treat the six additional union representatives as trespassers. 

[1]Fred Meyer Stores, Inc., 359 NLRB No. 34 (December 13, 2012).