Revealing mistake: When Squad 51 leaves Rampart, in the first interior shot facing Roy as John says, "Yeah, I'll tell ya, when it hits ya... Boy when I met Valerie it hit me," through the back window we can see movement at the back of the squad. But in the shot facing Roy when John says, "So that's that... That's exactly how it is," through that back window we can actually see a crew member's face (sideways), who has a mustache, right behind John's head, as he's riding on the back of the moving squad. It's so funny, have a look.
Other mistake: During the structure fire, when Engine 98 rolls up we can see the large red vinyl adhered to the top of the engine's cab. Then when Truck 106 rolls up we see its company number 106 on the door, but in the overhead shot we can see the number 8 on the roof of the cab, not 106. This presumably means that the door has one of the prop dept's magnetic decals imprinted with 106, and the roof still shows the Truck company's real number.
Other mistake: After finding out about Valerie's three kids, when Johnny and Roy are sitting in the station's locker room at the start of their next shift, there are a few moments during this conversation where Kevin Tighe softly mouths some of Randolph Mantooth's lines as Johnny is speaking, particularly when Johnny says "Beautiful chic."
Visible crew/equipment: When Johnny jumps from the third floor ledge onto the lifenet, in the shot of his actual jump we can see a cameraman wearing jeans at the bottom left corner of the screen, who's holding a handheld camera shooting footage while lying on the street. The angle of that footage can be seen just as Captain Stanley shouts, "Gage, you first," and also when they walk the lifenet over to their left, before Cap shouts, "Okay, Roy!"
Other mistake: After Station 51 is dispatched to the structure fire, just as Roy and Johnny start heading upstairs we see footage from 4x3, "Gossip," when Johnny brings the hose to the stairs at the chemical fire, but the line doesn't reach so he leaves the hose at the bottom step, and they continue up.
Visible crew/equipment: During the structure fire, after Roy and John climb out onto the ledge and the fire flashes in the exterior shots, the next interior shots we see shows stacked boxes falling as the fire flashes through the doors. The interior shots are from episode 3x7, "Promotion", during the structure fire, after the security guard is taken down the ladder. We can even see a crew member's hand pushing a lower box which causes them all to fall.