"Summoning Sickness" only stops creatures from attacking or being tapped to activate their own abilities that include the Tap or Untap symbol. Being "sick" doesn't prevent tapping a creature to pay the cost of another ability.
Treat each other with respect, in accordance with the MTR.
Being respectful of others isn't just the right thing to do, it's also a requirement of the Magic Tournament Rules.
Drown in Sorrow vs. Mutavault.
Drown in Sorrow only affects things that are creatures at the moment it resolves. Any creatures that appear later in the turn won't be affected!
Acolyte’s Reward and changing Devotion.
Acolyte's Reward only checks your devotion to white as it resolves (not before, not after). So the amount of damage that will be prevented/dealt is locked in at that moment.
Their Bile Blight vs. your Pack Rat – Killing your own creature can be good for you.
Bile Blight needs its single target to be legal in order to resolve and do anything, so you have the chance to remove that target in response in order to save the rest of your creatures that share a name with it!
Courser of Kruphix and Sphinx’s Revelation.
When drawing multiple cards at once while playing with the top card of your library revealed, each draw is interrupted by revealing the next top card before drawing it.
The importance of shuffling (piles are not enough!)
Piles are not random, and therefore aren't actually shuffling. You must use a shuffling method that actually accomplishes randomization (side/mash shuffling, riffle shuffle).
Courser of Kruphix and Shock Lands from the library.
When playing a shockland from your library, it doesn't actually leave the library until after you've made the decision about whether to pay two life or not. That means you don't get to see the next card down before making that choice!
Brimaz vs. Silent Arbiter.
Against Silent Arbiter, Brimaz can attack by himself, but you still end up with 2 attacking creatures.