Recently I came across the following error when working on a group Unity3D project:
Unable to parse YAML file: [could not find expected ':'] at line 2
We had no yaml files (that we knew of) and Unity could still build and run the project so this error seemed a bit odd.
It turns out that the error was caused by git adding text to highlight a conflict in some of Unity's .meta files. So fixing the error was just a case of finding the meta files and resolving the conflicts.
I used this ruby script to find the files that needed to be fixed. Just place the script in the root of the Unity project and run it. It'll print the locations of the broken files.