If you are using ArcGIS Server, with WFS capability enabled, then pay attention to the help files, and set the "Enable maximum number of feature" to a reasonable level. The default is 1000.
If you set the value too high, it will write large files to the server output directory, and in our case, crash the server. Our server ran out of memory, the disk thrashed, and the gluster share had a melt down and took the whole site down. This is server that is used to handling millions of requests a week.
Our fault - we should have paid more attention and not set this value as high as we did with a more testing.