Mixed up display after screen replacement

Hi, I replaced the screen/digitizer for a colleague of mine. I quickly began to wish that I had never agreed to do it, but that is another matter...

Everything went generally well, but now when he brings up a complex screen view like the gallery, the display gets all mashed up in diagonal stripes. Going back to a simpler screen view and everything goes back to normal.

Could this be a loose connector, or is it something more serious? It's a blooming dog of a phone to open up, and there are so many flipping connectors, so I'm reluctant to simply go fishing around inside.