Google announced it is implementing new crawl error report in webmaster tools to help webmasters understand crawl error that are being displayed for redirected URLs. There has been much confusion from webmasters over what exactly the crawl errors meant for redirected URLs and the way they were being displayed within Webmaster tools. The confusion emerge because when a webmaster was redirected from site A to site B, if there was a problem on site B in regards to the redirected, it would display a crawl error that it was from site A, when what it really meant was that there was as error redirecting from site A to B.
Having thousands of 404 errors, especially ones for URLS that are being indexed out linked to by other pages pose a potentially poor user experience for your users. If they are landing on multiple 404 pages in one session, their trust for your site decreases and of course leads to frustration and bounces.
Below are list of crawl Error Alerts