[miguelpais]

The biggest illusion of progress bars is when they don't accurately represent the time left for something to complete. Like when you wait 7,5 minutes for the bar to reach 75% and then it suddenly jumps to 100%, instead of taking another 2,5 minutes to complete (that is frequent on installation processes).

Of course, when it comes to download progress bars it's not possible to make it accurate without making it bigger as the download speed drops and making it infinite/disable it when the speed is 0KB/s. But in other offline tasks the progress bar is frequently useless to capture time remaining for the completion of something.

That's probably why a time remaining label is added to them.

[billswift]

Firefox is worse; it often immediately goes to about 75%, then just sits there, then jumps to the end; it is the most useless progress bar I have seen yet (and I have been using computers since Windows 3.1).

[narag]

A small file could cause this. The browser starts to download the file to a temporary directory, while the user is choosing the final location, so when the progress bar appears, most of the file is already there, then the rest of the file arrives at the same time that the system is creating the new file and copying the contents from the temporary file to the location chosen by the user. Disk file operations can block for an instant. When the bar is to be updated again, the download is complete, so it simply vanishes.