"If verification fails, delete index reference" is enabled, or anything else from viewtopic.php?f=23&t=5314&p=30466#p30466 is being done. Data about a page is to be deleted. But the page is actually not available not because it was updated, but because the server is down, or broken, or being blocked (censored) by mistake or otherwise temporarily. It is likely to go up soon, or the block (censorship) is going to be lifted.
A list of strings to detect such cases, and ways of detection (for example, delete only after the URL has been accessed at least 3 times with an interval of at least 3 days, but at most 30 days in sum).
Problem with the proposal:
1. It can be used to increase DoS spam.
1. Only use for cached pages?
Another potential problem:
2. It can be seen from outside that somebody is searching repeatedly for something intentionally censored.