Please provide more details on this...

1) Please mention which specific cleanup tool triggered this error.

2) Please mention how the error displayed. Was it for example a little warning attached to the top of the main cleanup tool results, below the main results, a part of the results, reported tangentially (e.g. in a separate error email), or a big error that stopped the process in it's tracks?

I believe this would be the broken URL check, and only happen if you had deleted a topic you linked to somewhere. It would have happened in an error message notification, not on the screen itself. As that is a likely thing to happen, I will disable this error message for topics. I will also make it more clear, as it is truncating the full page-link which should really of referenced the topic ID involved.

I did several at once, so not sure, and yes it was a system email. I don't tend to link to topics at all, only pages, so I think we can rule that out. I will see if I can trigger it again and let you know more details.

Something was definitely linking to a topic. It could have been something very casual, such as just a topic link pasted into a forum post somewhere. There is no actual bug here, so no need to keep us informed.