Rubick spellsteals Walrus Punch, pretty normal. Rubick uses Walrus Punch by firing a projectile that's essentially himself to uppercut the enemy? Hillarious.
When Tusk uses Walrus Punch normally, the text "WALRUS PUNCH" appears in red letters above the unfortunate target. What happens when Rubick uses it? "STOLEN PUNCH" (or, if you hit Tusk himself, "SUCKER PUNCH").
An E-sports example. In the wild card game (the winner of which would go on to play in TI 3) between Rattlesnake and Quantic, there was a moment where Sven (played by Quantic's Sockshka) ambushed Chaos Knight, Gyrocopter and Beastmaster and attempted to stun them all with storm hammer so that his teammates could catch up to them and possibly kill them with few losses. In a very unfortunate case of a misclick, Sochshka cast storm hammer not on Chaos Knight, not on Gyrocopter, not even on Beastmaster, but on... Beastmaster's Boar, stunning absolutely nobody but the boar. This ended up getting Quantic team wiped with the exception of one. This moment can be seen in its full glory in this video.
Another example sort of from TI3: On the first day of the competition, an "all-star match" was held between two teams voted on by the players. Given that there was nothing at stake, the players took it upon themselves to goof off in the most spectacular way they could. Highlights include the members of Alliance (Loda and Akke) and Na'Vi (Dendi and Puppey) trash-talking their teammates, who were on the opposite team for this match, a completely ridiculous draft, which was made by having the team's "Captains" picking the other team's picks using heroes that are almost never used in professional games, such as Sniper, Omniknight, Skeleton King and Axe (the Dire side actually had an all-melee team), and Dendi celebrating killing Puppey (his captain) by getting out of the booth and dancing in front of Puppey. You can watch it here, starting at 11.02.30.
One of the changes to Bloodseeker's Thirst ability in the 6.79 patch was causing it to remove the speed cap when active. This had predictable results.
Fountain Hookingnote For the uninitiated, the technique involves combining Chen's Test of Faith ability that sends an allied hero back to their home base with a perfectly timed use of Pudge's Meat Hook ability to pull an enemy unit toward Pudge. The result: a helpless enemy hero flying over otherwise-impassable terrain back to the fountain-hooker's base, where the base's automatic defense structure kills him very quickly. is awesome, flashy and all. And sometimes it backfires...note Fighting a fully stacked Chaos Knight without a disabler nearby is a really bad idea.
Wraith King's new lines. Some of them are outright references to CASPLOCK LEORIC.
Windranger's lines for dying, killing someone, and others are chuckle worthy
Upon dying: Why do you hate gingers so?
Killing someone: Pain, meet Ugly. Ugly, Pain.
Casting Shackleshot: I now pronounce you man and tree.
Sven Lampshading the fact that he's using Daedalus, a crossbow, to increase his damage.
Sven "Daedalus! This is going to be absurd."
This video shows what happens when someone who plays supports attempts to play a hard carry, such as Anti Mage. It ends with the Mage using Manta Style note creates two clones of you that take increased damage and going to attack Sven, who then kills Anti Mage and his illusions in just two hits, which causes the Mage's team to lose the game.
Phoenix's spell descriptions are normal, the sub-spells though?
In this video, Puck spends about one minute attempting to both bait and dodge Invoker's Sunstrike while trying to get back to the base. He fails miserably just as he is about to get to the fountain.
This video involves about why someone plays Sven and every of his awesome display of Triple/Ultra-Killing gets a replay accompanied with epic chorus. But then, for his last 'awesome' moment, after committing his Triple Kill, Sven got killed by a Tower, and the music abruptly screeched to stop at the Epic Fail.
The Stanley Parable announcer. There are probably too many to list, but one standout is him calling almost every purchased item the "poor man's Divine Rapier". Have a listen.
Some heroes have voice responses for when you buy items or use runes they don't need.
Anti-Mage: Blink Dagger? Really?
Viper: Orb of Venom. I need this why?
Nyx Assassin: Blade mail?
Nyx Assassin when picking up an invisibility rune: They have runes for this?
Riki when picking up an invisibility rune: This is redundant.
Queen of Pain: Blink dagger? That was quite unnecessary.