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YOU CAN’T CUT BACK ON FUNDING! YOU WILL REGRET THIS! is a catchphrase originating from the video game Sim City 2000, where it is shouted towards the player (the mayor) by the transportation advisor if he attempts to cut transit funding.
Released in 1994, SimCity 2000 was the first major sequel to the original 1989 SimCity. Critically acclaimed and award-winning, the game is still regarded highly to this day, and PC Gamer magazine ranks it as the best SimCity of all time.
The board of advisors are often considered an annoying aspect to the game because they repeatedly pop-up to give advice and complain. The transportation advisor is one of the more memorable advisors due to his over-the-top screams in all capital letters. TV Tropes lists this part of the game as a “What the Hell, Hero?” trope.
Yet other times, an author will drop some gray into his otherwise flawlessly good guy hero… and have others notice. This can add guilt and remorse to a character as they feel shame for their evil actions, and legitimately have to fight to restore their name, undo the harm, or simply live with the guilt and shame of it. In this case, you can expect to hear some variation of “Haven’t you done enough already?” 
On the Internet, one of the first notable uses of the catchphrase was in a YTMND titled “YOU CAN’T CUT BACK ON FUNDING!”, posted July 26th, 2005. It featured a single static picture of the advisor’s quote with trailer music from The Lord of the Rings: The Two Towers, a re-orchestrated version of the Requiem for a Dream theme (see also Fire Spirit theme).
After the original YTMND, numerous spin-offs and variations started to appear, with the most popular being the also simply titled, YOU CAN’T CUT BACK ON FUNDING! YOU’LL REGRET THIS!, first published on the 29th of May 2007, this one featured a more elaborate background as well as a sound clip of King Acorn (Tim Curry) from Sonic The Hedgehog SatAM shouting “You’ll regret this!”.
Later it began being used in images that could be related to a lack of funding like an unfinished Death Star, or just to express a more general outrage. The catchphrase can been found in several Something Awful threads.