No missable achievements (plus 50 unknown)
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Pass the toilet paper to the scientist in need.
Ruin the microwave casserole.
Headshot 20 enemies with the .357 magnum.
Get overtaken by a Headcrab's ample anterior.
Perform surgical caffeine extraction from 20 machines.
Kill 20 enemies with the Hivehand.
Use the alternate path in Office Complex.
Kill a Headcrab using fire.
Overcharge it anyway.
Interfere with the alarm and laptop in Anomalous Materials.
Kill an Assassin with the .357 Magnum.
Use an entire clip of Gluon ammo in one continuous shot.
Kill 5 enemies with one grenade.
Kill 10 Airborne Headcrabs using the Shotgun.
Get killed by your own Snarks.
Refuse to insert the specimen into the Anti-Mass Spectrometer.
Kill a Barnacle with a Satchel.
Kill an Icthyosaur with either the Tau or Gluon.
Kill 10 enemies with a reprogrammed Sentry.
Kill 10 enemies with Snarks.
Drown in Lambda Core's coolant.
Conduct all of the questionable research experiments.
Help the Barnacle and Headcrab find true love.
Kill a Controller without killing any of its mind-controlled Vortigaunt slaves.
Kill a Marine with their own grenade.
Accept G-Man’s offer of employment.
Send the Hidden Hat to Xen.
Cook a Grenade for too long.
Decline G-Man’s offer of employment.
Die in the AMS Beam before starting the experiment.
Don't kill any Vortigaunts in the Borderworld.
Escape the Hangar in Surface Tension without alerting the Soldiers.
Kill 10 Enemies with Tripmines.
Assemble all the possible Scientists and Guards at the end of Office Complex.
Get whacked into oblivion in the ledge fight.
Skip the opening Xen island with a secret jump.
Deliver the finishing blow to Gonarch with your Crowbar.
Escape the Gargantua’s Lair without taking any damage from Gargantuas.
Take out all three Healing Crystals in Nihilanth’s Chamber at the same time.
Defeat Nihilanth in under 4 minutes.
Complete Interloper without taking any laser damage.
Find each member of the missing research teams.
Kill all Protozoans in the Borderworld.