A scope creep is usually a requirement that was omitted in the initial scope and now creeps on you.
Here's an example:
We all know that every form on the web needs validation (for example, email validation, phone validation, etc...). Your project is to create an online shopping cart for a website, you have included everything in your initial scope (or maybe thought you did). Few weeks into the project, one of your programmer asks you, what about validation, it is not mentioned anywhere in the scope. You realize at this point that you have omitted a critical task from the scope, so now you have to adjust your schedule to accommodate this out of the blue requirement that should have been there in the first place.
Needless to say, the sponsor and other stakeholders are not very supportive when it comes to scope creep.