Realy strange spectator mode behavior while spectating a minecart

Only in singleplayer:
If you go into spectator mode (/gamemode 3), then spectate a minecart by left-clicking it. After this you change your gamemode again (/gamemode 1) and left-click/punch forward, you get an error screen saying: "Connection Lost - Attempting to attack invalid entity", but then you get to your server list. (That's strange because I started the world in Singleplayer.)