Doors

This is usually due to an incorrect orientation of your entry/exit parts.

It's very important you enable the orientation indicator (pictured). This allows you to see which way your part is facing.

The 'F' indicates forwards, and the little green arrow indicates the top.

It is vital that the orientations of your entry/exit parts face the same way. 

In this wrong example, the rooms will end up spawning inside each other (bad!) 

You can position them either way, but the important thing is that they all face the same direction

In my tutorial video, I have them face the direction of travel 

For example, this is one of my corner pieces

The arrows show the orientation of the part. See how they're relative to where the player moves

And a longer straight section for example