How clients tend to not understand how much work developing a "simple" or "small" feature can be.

Hey add live chat, that's easy right?

Reminds me of that xkcd that goes something like:

When a user takes a photo, the app should check whether they're in a national park...

Sure, easy GPS lookup. Gimme a few hours.

...and check whether the photo is of a bird.

I'll need a research team and 5 years

@tytho exactly right! I never want to see the "Should be easy" or "Should not take that much work" sentence from a client xD