Question: I don't know why I cleaned the project, but I did, and now one of my activities is reporting 50 errors, like:

```
    setContentView(R.layout.activity_settings);
    // activity_settings cannot be resolved or is not a field

    CheckBox check_sound = (CheckBox)findViewById(R.id.settings_sound);
    // settings_sound cannot be resolved or is not a field

```

And so on. This was working fine before I cleaned the project and nothing has been deleted. Only one activity has errors, no other errors reported in any other file. Tried restarting, Build automatically is enabled, Project>Build and Project>Build All are greyed out.
What's going on?

Answer: 
Found the problem. For some reason, the first import in the problematic activity was `import android.R`. I don't know how/why it got there. I ran Lint and went through every one of the entries/errors/warnings until I deleted `import android.R` out of desperation. Suddenly everything started working properly again.

Of course when I Googled this solution, I found several answers but for some reason I could not find them before I found the answer myself.

I swear no single programming problem in any of my apps was even close to problems I had dealing with Eclipse/ADT. From a simple SDK update, to reinstallation, to nonsense like this,
This happens a lot, at least for me. Eclipse is kinda frustrating at times. 

This could be something as simple as a typo error in a layout file or a string file and it crops up after a project clean.

Unfortunately eclipse doesn't help and throws only an APT error rather than just showing where the error is. Rest of the times it behaves well by showing errors.

May be you could double check all your layout files and string files thoroughly. One way could be trying to see a graphic layout of your layout files, if all is well, a graphic preview is shown.