I would imagine that this happens under other circumstances too, but here's what I did:

1. Download Minecraft on a new computer
2. Open the launcher
3. Select Latest release
4. Play
5. Download bar appears on the bottom

The file size tracker at the beginning of the line (0MB/123.45MB) has a typo: the first number displays without a decimal point. (example: 10000MB/123.45MB instead of 100.00MB/123.45MB)