Scope creep
Also said as Mission creep
The project slowly growing during development, through a long series of small additions that each seemed reasonable, without the deadline or the budget moving to match.
No one decides to make the game twice as big. It happens one yes at a time, and nobody notices the date has become impossible until it is close.
Say it
“Every new request goes on the list with its cost in weeks, and for each one that gets in, something of the same size comes out. The date does not move.”
Where you’ve seen it
Star Citizen's 2012 crowdfunding campaign kept adding stretch goals as money came in. Ten years on, with more than 500 million dollars raised, the game was still in alpha.
Not to be confused with feature creep
Scope creep is about the whole project growing against its time and money. Feature creep is about the game itself getting more and more stuff, whether it needs it or not.