Karthus ultis. OH SH*T every1 has "Magic Thornmail" !@#$% Returns 2k damage... lulz

There are some things that won't be made to mirror DPS and Caster (rightly so, I haven't seen a caster skill that autocasts at a rate of attack speed, nor a DPS whose attack speed is .01 but nukes for 1.2k damage), and I prefer it that way. It'd be annoying and at times useless to get some of those items (+5 armor per skill cast? 300 hp shield for 6 seconds if below 30% HP?).

Plus, Thornmail+Magic Thornmail=automatic tank, doesn't matter who has it. Grab it on Eve or Twitch and laugh at their pathetic attempts to focus you.
or
Someone have Thornmail? Your mirror mail will cause damage reflection on their damage reflection (granted both will be reduced considerably because their damage type is the kind your armor is meant for)
And my favorite
Two people have mirror mail, and one casts a spell. Infinite damage reflection loop. Seriously:
100 damage-30 damage-9 damage-3 damage-1 damage
Now imagine that with much higher damage numbers.