In Round 8 of Pro Tour Aether Revolt, an incident occurred in which the active player indicated that they were entering combat and passed priority. The opponent passed priority back, at which time the active player attempted to crew a vehicle and resolve a trigger with a target (Weldfast Engineer).
It was ruled that through the players' actions, they were in the declare attackers step and were unable to perform the actions that the active player wished.
This is the correct interpretation of Magic Tournament Rules (MTR) section 4.2, which deals with shortcuts—
"A statement such as 'I'm ready for combat' or 'Declare attackers?' offers to keep passing priority until an opponent has priority in the beginning of combat step. Opponents are assumed to be acting then unless they specify otherwise."
The intricacies of communication around the combat step are complex, and it's important to protect the non-active player from ambiguity in communication that can be exploited by the active player. Our tournament rules are always under review and revision, and we will be examining this rule further after this event.
You can read the full Magic Tournament Rules at this page.