The steps to reproduce:

1. Go to global template in Administration console -> Configuration -> Global Templates.
2. Create a new template contains the following markup:

   {toc}
   
   h1. test1
   test test test test
   
   h1. test2
   test test etest test
   
   h1. test3
   sdlgkmldfkmg lkdmf glkdmfg lmgfd
   

3. Click button Preview.

You should see error message like what I have attached.