Scratch

I know how to make a ENDLESS scroller!

Make a variable called <{ scrollx }>

Now you make a terrain, and put a script on it
<when green flag clicked>
<forever>
<set x to( ((<{ scrollx }>  <+>((460  <*> 0 ))  ))
<end>
The number of the script depends of the terrain you have.
<when green flag clicked>
<forever>
<if><abs( <x position><( <>>460  )>
<if>
<hide>
<else>
<show>
<end>

Now for the background:

<when green flag clicked>
<forever if><( <{ scrollx }> <<> -915  )>
<set{ scrollx }to( -120
<end>

<when green flag clicked>
<forever if><( <{ scrollx }> <>> -10 )>
<set{ scrollx }to( -900
<end>

<when green flag clicked>
<forever if><key[ right arrow ]pressed?>
<change{ scrollx }by( 5
<end>

<when green flag clicked>
<forever if><key[ left arrow ]pressed?>
<change{ scrollx }by( -5
<end>

And that's it!