For example, if you conduct a site:www.bbc.co.uk and restrict it to "pages from the UK", you get zero results. I was finally able to reproduce it and here is a screen capture.
If you remove the "pages from the UK" filter, and conduct the search again, it does bring back results.
If you use the "pages from the UK" filter and add a keyword to the end of the command, it works again. For example a search restricted to UK pages on, on bbc.co.uk for "bank" results returns many results.
SEOptimise has other examples of the problems with this site command search at Google.