[HK part (Weapons Protocol) - only from Story Mode False Emperor?]

You can get the HK part in story mode or hard mode. In story mode, right after you defeat Revan and get his drop box, there is something that looks like a holotransmitter sitting on the ledge to the right of the final computer. If you click on it, you get the HK part.

[List of Missing or Bugged Codex entries from all Planets(Imperium)]

For the Mutated Colicoid codex, when you encounter the two drones, ensure that you have your companion dismissed and DO NOT use an area of effect attack. Kill them one by one and you will get the entry, if you kill them in one shot with an AOE you get nothing and the respawn time is horrendous!!

[History of Tatooine]

There are two Krayt dragon skeletons, one is next to an instance cave, the other one has a long bone next to it that is the codex entry

[Funniest Boss Fight Ever!!!! (LvL 40 Sorcerer Spoilers)]

I tried to purposely do this on my 37 Sin and Overload simply never worked no matter how many times I used it

 

I got killed three times in a row, then read the overload advice here and stood next to the pit and it worked on the first try:D