TYPO3 Core

Well it is happening quite often, looks like every 30th page of our site is affected. Still on 6.2.3 though as upgrade will occure next week, but I do not find any bug fix mentioned in the 6.2.4 release notes.

I wonder why the compressed files are missing, e.g.
typo3temp/compressor/merged-cc411e5ddb8e7234ea9ab39dc4366fca-86ef4a25bd01a8a8926ab15ab691676b.css?1405674327

shouldnt these files be the same on each call? Why create new named ones when the cache is cleared and what is deleting the old files? Could it be that if a page is called with different calls (we use simulate static documents) so e.g. abc.123.0.html and def.123.0.html that the first merged css is deleted?

I'm not aware of a feature that is actually removing files from typo3temp (except the Install Tool).

The filename of the the merged files is created from these parameters by md5() hashing them:
$filenameAbsolute . $fileStatus['mtime'] . $fileStatus['size']

So new files are only created if the filename changes or if the file is modified.

Then the question is why the file is not generated sometimes (it definately did not change anything on the pages that should trigger new MD5 hashes).

Maybe because it is not necessary to create the file if the entire pages comes from cache anyway. TYPO3 does not expect that the files are deleted without clearing the cache as well.

Well the file should not be deleted by anything as you wrote above (I definately did not delete it manually or using the Install Tool). So either something is deleting the cache file or it was not created properly.

As I said there is no logic I'm aware of that deletes those files. Also not when clearing the cache.
If you do delete such a file manually, then you will need to clear the cache.

If the file could not be created, then you should find some warning in the logs. (either TYPO3 log or webserver/php log)