System Information
Win8.1x64, gtx580

Blender Version
2.79

Short description of error
When scaling animation curve to a negative value to reverse it, until the mouse drag is introduced, the first vertex loses it's vectors, which gets saved after scale confirmation.

Exact steps for others to reproduce the error
In graph editor:

• Select all animation,
• press S, -1. -> Until the mouse gets dragged, the new rightmost vertex loses it's vectors.