Argh, edited out the part number apparently.
It is the 4051 multiplexer, 8 channel analogue.
The buttons on the shield all work through the same analog pin, the signal can only be read when the multiplexer is set to that pin, obviously.
As for schematic, I don't have a schematic I'm afraid, not exactly sure why it'd help (will add one tomorrow, its late here).
Safe to say, I have a digital output, and a analog input attached to the 4051 multiplexer.
When rapidly changing the pinMode to switch between writing the digital output, and reading the analog input, the LCD displays garbage, so something isn't playing nice.
PS: The issue only occurs when switching rapidly.