Arduino Uno + Wifi Shield + PS2 Keyboard + LCD Display
Sorry I do not have enough reputation to place a comment. 

@Valentin from the link, you provided, to the wifi shield it states that it uses pins 11 ,12 and 13 on the UNO which will give you a conflict on pins 11 and 12 with the LCD. Not sure why that manifests as an apparent keyboard error though: 

From the linked page:


  Arduino communicates with both the Wifi shield's processor and SD card using the SPI bus (through the ICSP header). This is on digital pins 11, 12, and 13 on the Uno and pins 50, >51, and 52 on the Mega. On both boards, pin 10 is used to select the HDG204 and pin 4 for >the SD card. These pins cannot be used for general I/O. On the Mega, the hardware SS pin, >53, is not used to select either the HDG204 or the SD card, but it must be kept as an output or the SPI interface won't work.


Again sorry this is not a full answer but thought it may help.