The 8-debuff limit

[Concillian]

5) Debuffs will allow a newer debuff of a lesser rank to overwrite an existing debuff -- unlike player buffs, where a player attempting to overwrite an existing stronger buff will be told that "A more powerful spell is already active."  I haven't tested this.

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It used to give the more powerful effect message, but I think MC was one of the big reasons they changed that and improved sunder armor.

Scenario 1:
MT has aggro well under control, MT has (old) improved sunder
MT dies due to unexpected DPS spike
Secondary tank tries desperately to secure aggro with un-improved sunder:
"A more powerful effect is already active"

Guaranteed wipe

Scenario 2:
MT has aggro well under control
Some rogue decides to (or accidentally) use Expose Armor
MT cannot sunder for the duration of the Expose Armor because "A more powerful effect is already active"

not a guaranteed wipe, but it will give the MT fits if the rogue continues in this vein.

I think these were the main motivating factors to the recent changes to how the effects wipe each other and to improved sunder.