Gary Illyes said some search engines can be confused by seeing double slashes in a URL. Previously, Google suggested not to use double slashes in your URLs but here Gary is saying that technically pages load with double slashes but with some search engines may have issues crawling URLs with double slashes.
Gary explained this in the latest Google SEO office hours at the 5:46 mark in that video. He was asked, "What is the SEO impact of using double slashes in a URL?"
Gary answered, "Well, from a Puritan perspective, that's not an issue. If you look at RFC, I think, 3986 Section 3. The forward slash is a separator and is okay to appear in the URL path as many times as you like even repeatedly. From a usability perspective it's probably not the greatest idea and it may also confuse some crawlers."
Here is that video:
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