To reproduce:

1. Download attached project “CurveTonsyt.zip" and open in Unity
2. Open "SampleScene" scene
3. In Hierarchy select "Particle System" gameObject
4. Select "Size" curve in the "Size over Lifetime" module in Inspector
5. In the Curve Editor select a Curve key
6. Click on the key handle and hold the Mouse Button down
7. Hit Delete key while still holding the key handle
8. Keep moving Mouse around while still holding the key handle

Expected results: No Warning or Error are printed in Console
Actual results: Console spams hundreds "IndexOutOfRangeException: GetKey" Errors with each Mouse movement

Notes:
- This issue appears both on Windows and macOS
- In Unity 2017.4 I get "IndexOutOfRangeException: MoveKey"
- On macOS, if you do not have a Delete button press fn + Backspace
- A Video is attached in Comments section

Reproduced on Unity 2017.4.0f1, 2017.4.20f2, 2018.3.7f1, 2019.1.0b5 and 2019.2.0a6