Progress bars. They're better than nothing, but who among us hasn't stared at a misleading, confusing or wholly inaccurate progress bars and wondered, "Why can't anyone make these more accurate?" Popular Mechanic's John Herrman answers that question.
Image remixed from CSS-Tricks.
The short answer: Many variables are involved in how long a process or download is going to take, and your computer isn't that good at calculating unknowns. Herrman's full post is an interesting read, but the main takeaway: Progress bars really are better than nothing (imagine an alternative where you wait for a result with no indication of progress until a process is complete), but you shouldn't set your watch by them (which you probably already knew).
Got a most-hated progress bar? Commiserate in the comments.
Why the Progress Bar Is Lying to You [Popular Mechanics]