Steps to reproduce:

1. Open attached project and "test.unity" scene;
2. Select Terrain gameobject in the Hierarchy view;
3. Go to Inspector -> Terrain Component -> Place Trees tab;
4. Select tree and click Mass Place Trees button;
5. In the opened window input any value in range of 8-19 digits;
6. Click Place button;
7. Notice that validation error, telling that "Mass placing more than 1000000 trees is not supported" is displayed;
8. Input value bigger than 19 digits (20+);
9. Click Place button.

Actual result:
The validation error is not shown when inputting more than 19 digits (see attached gif video).

Expected result:
The validation error should be shown when inputting any value bigger then 1000000 (see attached expected.png screenshot).

Reproduced on 5.3.0b5, 5.3.0b2 for both OSX 10.11 and Windows 8.1.