As a designer, what I love about early playtests is the ability to make changes as we go along during a test. When you go into a playtest, you should know that there might be significant rule changes made during the game.
There might be times when the game is broken and there is no point to continue the test as it is. The designer may choose to stop the game there and debrief with the group. They may decide to stop that game and start over.
Whatever it is that happens during a test, just go with it. Games can take much longer than you think they should, the rules can change on the fly, there could be players jumping into and out of a game, the designer could get pulled away and leave the group to play a turn on their own. Be patient and go with the flow.
If you have any comments or questions, leave a comment here or email Chris at firstname.lastname@example.org