So I was just galloping on my horse and I came to what I thought was an arch but strangely I could see the entire town of riften through it. I realised it was the big gate at the back of riften. I went to it, with the aim of obviously getting off my horse and selecting the door to go through it. Instead I galloped right through the door as if it wasn't there and I had an open cities mod on . I'm on PS3 and this post 1.5 patch.

Sweet.

More than likely you clipped yourself through a mesh join. When two meshes are put together, they are supposed to create a tight seam. However, if you put the correct pixel of your character at the right place on the seam, you will go through the mesh. Usually what happens is you fall through the floor into the void, but you can also glitch yourself through a wall.

I have also noticed that exterior doors are not always shut, so that may be way this happened, but the same explanation as above applies, only these are doors and not wall meshes.

Bottom line, you can probably head butt that gate for the next 6 months and not do this again. Or, you could hit the right spot and fall through again.

I wonder if Beth will ever fix this, as myself and other players have taken advantage of this bug to create a sort of "tcl" for consoles.

That is probably why they are there, so console gamers can see for themselves there is nothing magic behind the scenes.....