When Matthew is proposing to April, and all the doctors come out to watch, Callie is with them, however moments before she was operating with Hunt, Webber, and Ben. She would likely have still been in the operating room when the proposal happened as Hunt knew it was almost time and had kicked April out of the operating room.
When April and Matthew start kissing right after his proposal, April wears gloves. A moment later though, the gloves disappear and April wears her engagement ring.
When Ben Warren and Dr. Owen are in the elevator, Ben has normal clothes, however when they run to the OR, Dr. Warren has his scrubs, ID, cap, and mask. Having time to change would not have been possible, as they needed to run there to keep the OR from exploding with sparks of fire.