On one project, customer was very set to make smooth, accurate and with "modern animations" progress bar. It took almost 90% of time and resources (I had to split one progress bar thru few threads and wait for calculations from this threads, so progress bar is slowing down whole application). And still it's accuracy is only a little bit more, than many, many "smoothing" tricks. It wasn't worth the time, but experience from designing and coding that was something priceless.