Des Traynor (off of the fantastic Intercom blog) highlights some prime examples of iceberg features – those requests from clients that seem like simple and obvious functionality at first glance but opens up a can of worms when it comes to development and implementation.
Adding global search is often a popular request which is why, when I start a new project with a client, I use a framework that has native search functionality integrated (such as WordPress). Even if it’s not part of the original spec, I’m prepared for it in the future.