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Almost never, unless you are for some reason losing progress. A good example of a backwards going bar is the health-bar in a game (as mentioned in the comments). A health bar always depletes as you lose life. Notice however that a health bar is for a good reason not called a "progress bar". A health bar does not communicate progress to the user, instead it communicates something negative.

Almost never, unless you are for some reason losing progress. A good example of a backwards going bar is the health-bar in a game (as mentioned in the comments). A health bar always depletes as you lose life. Notice however that a health bar is for a good reason not called a "progress bar". A health bar does not necessary communicate "progress" to the user, instead it communicates a certain "status" to the user. If the bar drops it's perceived as something negative (especially if the bar cannot go up again).

Reading direction: the direction in which a progress bar fills, should in my eyes logically depend on the natural reading direction for a specific language. For RTL languages the bar should fill from the right to the left. However most RTL interfaces I've seen do strange enough not reverse the progress-bar. I'm not sure why this is the case, do people simply forget the progress-bar? Or is my theory based on nonsense?

Almost never, unless you are for some reason losing progress. A good example of a backwards going bar is the health-bar in a game (as mentioned in the comments). A health-bar always depletes as you lose live. Notice however that a health-bar is for a good reason not called a "progress-bar". A health-bar does not communicate progress to the user, instead is communicates something negative.

Note: I can't currently find a good source to back this answer with. But I do have an interesting paper that researched different types of forward going progress (linear progress vs accelerating progress etc.) : Rethinking the Progress Bar

Reading direction: the direction in which a progress bar fills, should in my eyes logically depend on the natural reading direction for a specific language. For RTL languages the bar should fill from the right to the left. However most RTL interfaces I've seen do strange enough not reverse the progress-bar. I'm not sure why this is the case, do people simply forget the progress-bar? Or is my theory based on nonsense?

Almost never, unless you are for some reason losing progress. A good example of a backwards going bar is the health-bar in a game (as mentioned in the comments). A health bar always depletes as you lose life. Notice however that a health bar is for a good reason not called a "progress bar". A health bar does not communicate progress to the user, instead it communicates something negative.

_{Note: I can't currently find a good source to back this answer up with. But I do have an interesting paper that researched different types of forward going progress (linear progress vs accelerating progress etc.) : Rethinking the Progress Bar}

The progress bar here could have been reversed, but that would communicate a completely different message to the user: The disk getting emptier. But in a forward filling motion it would communicate the correct message: "The progress of removing the files".

The progress bar here could have been reversed, but that would communicate a completely different message to the user: "The disk getting emptier". But in a forward filling motion it would communicate the correct message: "The progress of removing the files".