Unable to install CyanogenMod on Galaxy S2 i777
From the CyanogenMod Wiki:


  "Status 7" Error
  I have a stock AT&amp;T Galaxy S2 (i777) which had been updated to the latest version of Android from AT&amp;T. I followed the instructions for installing CM 10.1 but received the following error:

assert failed: getprop("ro.product.device") == "i777" || 
getprop("ro.build.product") == "i777" || 
getprop("ro.product.device") == "SGH-I777" || 
getprop("ro.build.product") == "SGH-I777" || 
getprop("ro.product.device") == "SGH-S959G" ||
getprop("ro.build.product") == "SGH-S959G"
E:Error in /sdcard/cm-10.1.3-i777.zip (Status 7) Installation aborted.

  
  I tried updating to the very latest version of ClockworkMod Recovery, but that didn't help. The solution I found was to edit the "updater-script" file located at cm-10.1.3-i777.zip\META-INF\com\google\android. I removed the first line from that file which was performing the assert and getprop statements. I was certain I have an i777, so I was pretty sure that removing this line would be ok. After removing this line of code I pushed the CM zip file to my phone and was then able to install ok.
  
  This page was last modified on 3 December 2013, at 22:42.


So if you are 100% positive that you have an i777, then you can follow these steps:


extract the update package zip file
navigate to */META-INF/com/google/android
open updater-script in a text editor like notepad++
remove the first line that does the "assert" checks. 
zip the folder back up, using only "store", no compression.