Why it's hard to notice
Every "can you also add..." seems small in isolation. Added up, these small requests can double the real effort compared to what was originally estimated, without anyone consciously making that decision.
How to keep it under control
- Write clearly in the quote/contract what's not included, not just what is.
- When an extra request comes in, respond with a clear impact: extra time, extra cost, or both.
- Keep tasks planned from the start visually separate from ones added later.
Not all scope creep is bad
Sometimes it's worth accepting a small request for free, as a goodwill gesture for a good relationship. The difference matters: do it consciously, not out of inertia, and not every time.
Bottom line
Scope creep isn't prevented by refusing every new request, but by making the cost of each request visible, for you and for the client alike.