SQL Parameter Sniffing - query execution time varies without reason

It was not a problem I encountered (maybe) but a blog post from Chris Brandsma from ElegantCode. Apparently, because of attempts by the SQL Server (not only Microsoft's, btw) to "optimize" a query based on input parameters, query execution time may vary immensely without any obvious reason. Imagine debugging that baby!

Anyway, here is the blog post, one important enough for Chris to repost it these last few days.