Darryl says that office safety training takes place once a year, or after an accident, and that "we've never made it a full year". However, in the episode "Boys and Girls", Madge, the female warehouse worker, erased the number 934 from a chalkboard, that had said "We've gone 934 days without a lost time accident". 934 days is over 2.5 years. "Boys and Girls" takes place only about one year before "Safety Training". (However, this could simply mean that previous accidents have not caused lost time.)
When Michael and Dwight are on the roof, the railroad cars in the background change positions during the conversation.
When Pam and Jim figure out Michael is going to jump from the roof, they try and convince him to safely come down. The camera pans to Michael on the roof and he is visibly talking, with no audio coming from the character.
Stanley doesn't see the watermelon that hit his car until the end of the episode, even though it is clearly visible when everyone comes out to witness Michael's "demonstration."