How to reproduce:
1. Open or create a new Unity Project
2. Open the ProBuilder Window, select "New Shape"
3. Select "Plane", set "Width Segments" to 789 or higher and "Length Segments" to 20
4. Press "Build Plane", wait for it to build
5. Notice the deformed Plane that was generated

Expected result: The plane is 10 by 10 and does not appear deformed
Actual result: The plane appears deformed

Reproducible with: 2017.4.0f1, 2017.4.14f1, 2018.1.9f2, 2018.2.14f1, 2018.3.0b7, 2019.1.0a6

Notes: Hovering over the object with "Edge Selection" selected also displays some anomalies. Attempting to alter the plane beyond simply moving it also seems to freeze Unity.