I don’t have access to the Alpha.
There were two bugs in particular that were… somewhat gamebreaking in Diablo 2:
Bug 1: Have two people in a party, at least one barb. One member stand still and have the barb whirlwind through them, back and forth, multiples times… fast. The barb can go elsewhere. Have someone hostile then shoot at the person whom was whirlwinded. The projectiles would go through them and homing projectiles just went back and forth. AKA “Projectile Invulnerability” in PvP. Still present in D2.
Bug 2: “Aura Stacking”. This bug was fixed in Diablo II some time ago. One paladin and one other. Have Dream runewords equipped on Paladin and fill the other’s inventory. Trade a potion to the other player and it would cause your Dream auras to flicker and stack. Could be done unlimited times.
Please make sure these aren’t present in D2R.