4 pipelining requests sounds okay.
After a bit of reading, it may be caused by looking for search results that don't exist. I've noticed something strange on google myself these past few months where if I begin browsing through the hundreds of pages of search results, the number of results pages mysteriously shortens as I go on. There aren't as many there as it makes out there are. If you try to jump to the later ones that don't exist, then it thinks you're causing trouble.
You know, I've seen that happen too. (Before all of this happened) At the bottom I would see a link that talked about omitted posts, I'd click them and my pages would go from 100 pages of results to 3
. Or I would click on page 72 in the Page 1 2 3 4 5...72 list and it would be blank or it would take me back to the first page.
This whole Google Sorry page issue has been happening with them since 2005. It is said that Google will not admit it's a bug.
Of course, I never knew of it until it happened this time around to me.