Dota 2 is an extremely complex game and always was, still is and will be filled with lots of different kind of bugs: some annoying, some game breaking, some abusable. There can only be a hope that the latter two types would be fixed as soon as possible. Although, it turns out that some of those can stay in the game for a quite a long time, suddenly used in official matches years after it being originally identified.
Phantom Lancer Phantom Rush — a mobility skill that allows you to close distance with the target in a very short time — can be manipulated to behave unintentionally. The bug itself is very easy to reproduce, creating lots of opportunities to gain unfair advantage. All you need to do is to use a Tango or a Quelling Blade on any tree during the start of Phantom Rush animation (after you issue attack command) to move to that tree at the Phantom Rush speed and then use move command to stop the animation (otherwise you will return to attacking the target you chose at first).
The bug was known to some players for a very long time, the dev forum post describing it in detail dates back to 2014 and was never fixed despite being known since a month after the skill's introduction. It took 3 years for it to finally surface in a professional games in an obvious fashion:
— PSG.LGD vs Vici Gaming in DreamLeague Season 8 China Qualifiers, on October 7. "Ame" escapes a gank using the bug.
One would expect intentional abuse of bugs to be universally banned in tournaments, but it is unlikely than "Ame" or PSG.LGD (who were the ones to intentionally use the bug during official matches) will be punished in any way, considering how forgiving (or some might call it unprofessional) Dota 2 admins tend to be regarding enforcing this particular rule and the fact that DreamLeague rules are unavailable to public in the first place. What would stop players from abusing bugs if there are no repercussions, even when used so blatantly during a match of Major qualifiers? Hopefully, with it finally being so publicly resurfaced, it will be fixed soon.
Update (October 8, 9pm CEST):
The 3 year old bug was fixed withing 24 hours after it was used in an official match.