Acid Reflex
Here is a fun coincidence between two Valve games! The most common achievement in both Half-Life 2: Episode Two and Left 4 Dead 2 are both called "Acid Reflex"!
I can't really think of any other achievements between Valve games that share a name like this (excluding Gnome Alone but that's literally the same achievement in all of them), but to be honest I can't blame them as the name is very good.
Both achievements are gotten by killing an acid-based enemy, the acid Antlion Workers for HL2: Episode Two, and the Spitter in L4D2.
Why am I mentioning this now? (Despite me mentioning it in passing before). Well, I am currently sick, and I have been getting a bit of actual Acid Reflux, and so this fact has stayed in my mind for a while.
Silly stuff like this is like the one good thing about being sick (nah probably not, it's just me being neurospicy)