If you have a linked sub report in Main Crystal Report, every time you open the main report, it re imports the sub report if it founds the sub report in the same location. This is despite the fact that Re-import Subreports when Opening Reports check box is unchecked.
This seems to me a bug. Similar bug in Crystal 10 has been reported on businessobject support web site.
This creates a annoying behavior, as Crystal report will remove the links between main report and sub report. Only workaround I have found is to rename the sub reports or move them to other folders, once they are imported in main report. If Crystal Report does not find the sub report in the original location, it does not re import the sub report.
I faced this issue when I changed the database driver of subreport and deployed in QA environment, still all my main report was referring to same old database driver for my subreport, after lots of investigation I discovered that I will have to reimport the subreport into all these 25 reports and redeploy all the report all over again, this is very painful and time consuming. If any body encounter similar problem and has better solution then please do let me know by posting in comments section.