This is a bit spaghetti right now, and this is compounding it further, so let's

look at all the cases:

Generic ScreenSelect, eg. ScreenSelectStyle:
 A non-joined player START press can join.  On successful join, discard the
 input; the start press that joins the player should not also make a selection
 on the menu.   Non-joined players can make no other inputs.  (ALLOW_DISABLED_PLAYER_INPUT
 is false.)
  - on start press
    - attempt to join the player; if successful:
     - play start sound (normally done by the derived class, eg. ScreenSelectMaster::MenuStart,
       but we're not going there)
     - return, so this start press that just joined the player doesn't also make a selection
    - any other button, return and ignore the press ("block input of disabled players")
  - on any other button press, just ignore it

ScreenJoin:
 Pressing start on a non-joined player joins the player, and unlike the generic case,
 it also passes the start press on as input, selecting the default option and starting the
 game.  (ALLOW_DISABLED_PLAYER_INPUT is true.)

 We don't have any visible menu selections, so other player inputs don't matter.
  - This screen assumes that a player can join, and that the first successful join
    will move on.  Pressing start and having it not join has never been tested or
    used, and in fact crashes: it'll be passed on as a selection, and
    SM_BeginFadingOut dies, because m_MasterPlayerNumber is invalid.
 
ScreenTitleMenu:
 Like SJoin, pressing start will select the current option and start the game.  Unlike
 SJoin, this screen has options; arrow key inputs on a non-joined player must be
 passed on, so the cursor can be moved.  (ALLOW_DISABLED_PLAYER_INPUT is true.)

This "always joins" assumption is bad, and was probably never intended.  Fix by
never allowing start on a non-joined player (unless the start just joined the
player, in which case he'll be joined by the time of this check).

stepmania/src/ScreenSelect.cpp

@@ -123,12 +123,18 @@ void ScreenSelect::Input( const InputEventPlus &input )
 			return;	// don't let the screen handle the MENU_START press
 	}
 
-	// block input of disabled players
-	if( !ALLOW_DISABLED_PLAYER_INPUT && !GAMESTATE->IsPlayerEnabled(input.pn) )
-		return;
+	if( !GAMESTATE->IsPlayerEnabled(input.pn) )
+	{
+		// block input of disabled players
+		if( !ALLOW_DISABLED_PLAYER_INPUT )
+			return;
 
-	if( IsTransitioning() )
-		return;
+		/* Never allow a START press by a player that's still not joined, even if ALLOW_DISABLED_PLAYER_INPUT
+		 * would allow other types of input.  If we let a non-joined player start, we might start the
+		 * game with no players joined (eg. if ScreenTitleJoin is started in pay with no credits). */
+		if( input.MenuI == GAME_BUTTON_START )
+			return;
+	}
 
 	ScreenWithMenuElements::Input( input );	// default input handler
 }
@@ -144,6 +150,7 @@ void ScreenSelect::HandleScreenMessage( const ScreenMessage SM )
 		 * if so, call ApplyToAll instead.
 		 * TODO: Think of a better way to handle this.
 		 */
+		ASSERT( GAMESTATE->m_MasterPlayerNumber != PlayerNumber_Invalid );
 		int iMastersIndex = this->GetSelectionIndex( GAMESTATE->m_MasterPlayerNumber );
 		bool bAllPlayersChoseTheSame = true;
 		FOREACH_HumanPlayer( p )