How to reproduce:
1. Open the attached project
2. Open the "Terrain.unity" scene
3. Copy "Max_Tocus" Game Object
4. Paste and move the Game Object in the scene View
5. Repeat steps 3-4

Actual result: Unity crashes.

Reproducible with: 2018.3.0b12, 2018.3.1f1, 2018.3.5f1.
Not reproducible with: 2018.3.6f1, 2019.1.0b3, 2018.2.0a5.

Notes:
Can't check older versions because the project gets corrupted

Crash happens after failed assertion:

Assertion failed on expression: 'edgeMask >= 0 && edgeMask < TerrainIndexGenerator::kMaxEdgeMask'

Wasn't able to get the stack trace from the crash dump or editor log files.