[Boost-users] [StateChart] event processing order in async machine

Hello, I've got an async. state machine (1.44, MSVC10), and sometimes I observe a very strange behavior: events being processed not in the order they are queued. I spent some time trying to investigate this issue, but due to the asynchronous nature of the processing, it's quite not trivial. So, before I start sinking too deep, I'd like to ensure I don't miss something trivial. Unfortunately, I don't have at the moment any self-contained code, but a very simplified model of my FSM is as follows. Assume there are 2 events: Ev1, Ev2. The both are queued to the FMS from the same thread, always in the same order: Ev1, Ev2. The FSM processes them in another thread. The FMS has some "entry" chain of states that defer the both events: S1 /[defer(Ev1, Ev2)] --> S2 /[defer(Ev1, Ev2)] and finally 2 orthogonal sub-states, one of them ignores the both events, and the other processes the both: ...S2 --> S3, S3_1 /[custom_reaction(Ev1), in_state_reaction(Ev2)] What I see is that *sometimes* the reaction for Ev2 is invoked *before* the reaction for Ev1. This "sometimes" really puzzles me: I'm absolutely sure that the order of queuing is always the same - so where can be a "race"? Any idea would be appreciated!