Uno R3's a4 and a5 not working after installing itead SD Shield 3.0
The shield has pull-up resistors for I2C/TWI on pins A4 and A5. Special care (resistor bridge) needs to be taken if using these as analog pins. Please see the schematics for more details. 

https://www.itead.cc/wiki/images/4/46/Sch_sd_shield_3.0.pdf

Cheers!