1. Create a project
2. Create two files containing a violation on something that only occurs in open files (for example, interfaces must start with I
3. Open both files

-- Note that you see both errors in the error list --

4. File -> Close Solution
5. Open the solution you just closed

Expected: For both errors to appear in the error list
Actual: Only error from the file that current the selected tab is in the error list

Very confusing - especially when I was using the "Open Documents" filter.