More Common Xamarin Forms Project problems

As a follow on from Resolving Common Issues Loading Xamarin Samples here are some other issues I seem to be commonly running into. These issues are not only referring to Xamarin samples, but to any Xamarin project.

Corrupt Xamarin-Android ZIP files

Another common problem seems to be with corrupt Xamarin-Android SDK zip files located here:
C:\Users\<Your Username>\AppData\Local\Xamarin\zips
You'll see an error message similar to "...zip file "C:\Users\<Your Username>\AppData\Local\Xamarin\zips\1FD832DCC1792D8ACA07FAC3259FC5A9.zip" is corrupt.

Deleting all the zip files in that folder will cause them to be downloaded again during the next build within Vs.  This will make the next build very slow for obvious reasons. Let it run its course, and resist the temptation to cancel it. It could take 20minutes or more. After they are downloaded and cached in that folder again, build speed will be back to normal.

Error "aapt.exe" exited with code -1073741819.

Another common symptom is build errors in the Android generated file Resource.cs file.
The following error will be generated if there are dashes "-" in any file in the Resource/Drawables folders.

Android SDK Updates

Generally if an error follows an Android SDK update it is most likely related to new build and SDK tools being used just installed/updated.  Like the above error the most likely symptom of this is errors from the generated file Resource.cs file in your Android project.
First try running a Clean build then a Rebuild.If that doesn't work the best bet might be to revert back to a previous version of the SDK Build tools. Also check forums.xamarin.com.

Error The "XamlCTask" task failed unexpectedly.

System.IO.FileNotFoundException: Could not load file or assembly 'Mono.Cecil, Version=0.9.6.0, Culture=neutral, PublicKeyToken=0738eb9f132ed756' or one of its dependencies. The system cannot find the file specified.
File name: 'Mono.Cecil, Version=0.9.6.0, Culture=neutral, PublicKeyToken=0738eb9f132ed756'
Resolved by Deleting the entire packages folder contents and allowing a build to restore all packages.