in a web farm that kind of error can happen if multiple processes try to write to the search index because only one process can get a lock on the file. The Lucene.NET search engine disk file system storage provider supports multi threaded apps but not multiple processes.
That kind of error could also happen after moving a site from one machine to another the permissions and owner of the file from the previous machine can cause such errors on the new machine.
If it isn't a web farm, I would delete the search index files and rebuild the search index. If it is a web farm I don't have a good solution, maybe best to use bing or google for site search in that case./rebuilding-the-search-index.aspx
Hope that helps,