To reproduce:
1) Create new project
2) Create Surface Shader
3) Create "Test.cginc" file
4) Open Surface Shader
5) Add '#include "Test.cginc"' to it
6) Open Test.cginc
7) Write some gibberish to make an error
8) Save and return to editor
9) Notice the error in the surface shader
10) Go back to Test.cginc
11) Delete all and save
12) Go back to Unity

Expected: The error disappears
Actual: The error stays

Reproduced in 2018.1.0a3, 2018.1.0a5, 2018.1.0a7, 2018.1.0b1, 2018.1.0b7
Not reproduced in 2018.1.0a2, 2018.1.0a1