What programming projects have you encountered that seemed easy at the beginning, but soon proved to be much more complex than you imagined? (Similar to this xkcd comic: https://xkcd.com/1425/)
For example, you wanted to do X, but in order to do X, you need to solve Y first, but to solve Y you need to (re-)write something from scratch, which has a much steeper difficulty curve, and so on.
Bonus question: Suppose you have limited budget, and thus you cannot implement that "seemingly simple" feature. How do you manage external (e.g. your employer's) or internal (your own) expectations?