If you use Magento EE and have enabled Solr for search, you may run into a few issues depending on how you customize your template. One issue is that the Enterprise Search engine for Solr does not properly handle resetting the result count when 0 results are found. The other two have to do with the way it indexes the data; it does not properly add some attributes to the index if you only have “Used for Sorting in Product Listing” enabled and/or they are the datetime attribute type on non-simple products.
The first one is relatively easy to reproduce but probably is not run into very often. On a website I was developing, the client wanted to break out the search results into individual categories and display the top few of each category results on the page. The page would then link out to each individual category to view all the results inside that category. The problem we found was that, if the previous category returned results and the current one it was outputting had none, it would still think that it had the same number of results as the previous category.