From Tamrielfoundry:

Class Abilities and Passives:

Assassination:

Death Stroke <Incapacitating Strike> and <Soul Harvest> On occasion the animation will happen however the ability is not actually triggered.
Assassin’s Blade <Killer’s Blade> and <Impale> – On occasion the animation will happen however the ability is not actually triggered.
Teleport Strike <Ambush> – On occasion the animation will happen however the ability is not actually triggered.
Haste <Focused Attacks> and <Incapacitate> – The attack speed increase of 30% doesn’t affect any weapons.
Mark Target <Piercing Mark> and <Reaper’s Mark> – The ignore 75% of target’s Armour and Spell Resistance doesn’t appear to work.
Mark Target <Reaper’s Mark> – The gain damage bonus effect does not appear to work.
Mark Target <Piercing Mark> – Allows you to see and attack targets who are sneaking however not targets that are invisible currently.
Pressure Points <Passive> – It currently gives .3% instead of 3% crit at level 2 for each Assassination ability slotted.
Pressure Points <Passive> – it is incorrectly giving 2% crit no matter how many assassin abilities are slotted. (Anyone have extra to add to this, noting the one previously changed?).
Shadow:

Shadow Cloak <Shadowy Disguise> – Approximately 25% of the time this ability does not trigger any of its effects.
Shadow Cloak <Shadowy Disguise> and <Dark Cloak> – On occasion you wont visually be invisible but you are to the mobs.
Shadow Cloak <Shadowy Disguise> – When utilising this ability with low resources you will instantly be removed from stealth. (Can anyone confirm its not the same as above?)
Shadow Cloak <Shadowy Disguise> and <Dark Cloak> – Your character becomes invisible however enemy players can see particle effects and thus be able to locate your player.
Shadow Cloak <Dark Cloak> – Does not remove bleed debuffs from npc or player sourced specific ones mentioned are Axe, Twin Slashes, Cleave. (Any more to add to this, or confirm/deny?)
Teleport Strike <Lotus Fan> and <Ambush> – Often when using Teleport strike it will have adverse effects to your FPS. (I have an Nvidia Graphics card, can anyone confirm on ATI,? I know this is broad)
Teleport Strike <Ambush> – On occasion the animation will happen however the ability is not actually triggered.
Veiled Strike <Concealed Weapon> – The 22% movement speed in stealth while slotted effect appears to suffer the dreaded “Once you’ve died you need to re-log or zone to rectify the issue”. Workaround: If you re-log or change zones it will force the passive effect to work again.
Veiled Strike <Surprise Attack> and <Concealed Weapon> – Often the animation and stun effect will occur but no damage is done to the enemy.
Path of Darkness <Twisting Path> and <Refreshing Path> – This ability appears to be affected by terrain, the magicka cost is spent however the ability does not trigger.
Path of Darkness <Refreshing Path> – Sometimes heals more than once per second, it has been document to happen anywhere up to 5 times per second on occasion.
Aspect of Terror <Mass Hysteria> and <Manifestation of Terror> – The base fear effect of 2 nearby enemies doesn’t appear to last the suggested 4 seconds all the time. It is often between 1.5 – 4 seconds.
Summon Shade <Dark Shades> and <Shadow Image> – The Shadow Image often stops attacking once the initial target it was cast on dies.
Shadow Barrier <Passive> – Doesn’t properly increase your Armor when coming out of Stealth or Invisibility for 4 seconds.
Siphoning:

Strife <Funnel Health> – On occasion can heal more than the suggested 2 ally players and sometimes it doesn’t apply to anyone including the player.
Strife <Funnel Health> – Doesn’t appear to be healing allies for the actual amount it should be, inconsistent heal effects.
Strife <Funnel Health> and <Swallow Soul> – Due to the ranged nature of this spell the projectile often takes several seconds to land on the intended target (even if in melee range).
Strife <Funnel Health> and <Swallow Soul> – Often once cast if the player was to turn away from the enemy target this ability will not do any damage to the intended enemy. (Nearly like it should be a channel from previous games)
Cripple <Debilitate> and <Crippling Grasp> – The dot effect of this ability does not work on mobs that are immune to snare effects.
Agony <Prolonged Suffering> – The added effect of this morph is meant to deal extra damage and this doesn’t work.
Siphoning Strikes <Leeching Strike> – Only gives an extra 2.4% health instead of the expected 4%.
Drain Power <Sap Essence> – The healing attribute of this morph is only increased by a maximum of 20% currently which is equal to one mob.
Drain Power <Sap Essence> – Is unable to perform a critical strike for both the damage and healing component of this ability.
Drain Power <Sap Essence> – Will not cast the heal effect unless the enemy is being damaged.
Magicka Flood <Passive> – Gives 8% max magicka at level 2 per siphoning ability that is slotted. This passive doesn’t detect Strife or Soul Shred but does detect <Funnel Health> a morph to Strife.

Weapon Abilities and passives:

Formatting <Formatting> -
Dual Wield:

Flurry <Rapid Strikes> and <Blinding Flurry> – Occasionally has an issue where the animation is trigger and you are locked in combat however the ability performs no damage. Workaround: To force Flurry to cancel its animation you should open your character info screen, the default key for this “c”.

Flurry < Rapid Strikes> – Does not appear to increase your attack speed. (Similar bug to haste maybe, all attack speed increase abilities are broken?)
Twin Blade and Blunt <Passive> – When you have daggers equipped you should gain an extra 5% crit at level 2 however this is not reflected in the character info sheet. (Can anyone confirm if this is actual vs. character info?).


Agility <Passive> – Is incorrectly giving the player 20% attack speed at level 2 instead of 10%.
Heavy Armour:

Vampire:

Vampirism Stage Bonuses <Buff> - “Your Vampirims abilities cost 60% less to cast” does not currently work for abilities using your Ultimate resource.
Mist Form <Poison Mist> – Appears to only be dealing damage between 1-1.5 seconds instead of the documented 0.5 seconds.
Undeath <Passive> – It is meant to provide damage mitigation when a play with is below 30% health, this is currently bugged and isn’t working.
Unnatural Resistance <Passive> – It is meant to provide health recovery in Vampirism stages 2-4, this is currently bugged and isn’t working.
Dark Stalker <Passive> – Suffers from the dreaded once you die you will need to relog and/or zone to gain the perk back.