Crowdfunding website Kickstarter has surprised some of its users by replying to complaints they made seven years ago.

Many focused on a book which attracted huge numbers of complaints about encouraging sexual assault.

Users who received responses to long-expired projects from 2013 took to Twitter to congratulate the company on its response times.

Kickstarter said the emails were "auto-generated in error".

"The emails folks received yesterday was due to an unfortunate human error while working on a clean-up task completely unrelated to the ticket from 2013," a company spokeswoman said.

"It's important to remember we are still a small team at Kickstarter and mistakes can happen."

But some of the complaints related to sensitive topics, annoying many who suddenly received a reminder.

The most prominent example cited online was about a book by a so-called "pick-up artist" that encouraged aggressive sexual behaviour.

The error dredged up an old argument over the book, which Kickstarter had refused to cancel on its platform, eventually publishing a blog post admitting a mistake on its part. But, by then, the creator had already received the money.

The automatically-generated emails told people who had reported it to Kickstarter years ago, that the book did not violate its guidelines.

Other Twitter users posted examples of complaints that received a reply: one involved a robot that could supposedly control a cockroach, reported for animal cruelty; another was a copyright complaint about a card game using established characters.

Most, however, dealt with the controversial book.

Technology journalist Holly Brockwell was among those who received the email erroneously, telling her the project had not broken any guidelines.

"Seven years, and big tech hasn't improved its response to sexual assault and harassment at all," she tweeted.

"God knows why they're drawing attention to their total lack of sense seven years on."

After Kickstarter replied to her explaining the error, Ms Brockwell said that it was " Just.... a bit of a kick in the teeth of an email, you know?"

"Totally. We completely understand," Kickstarter replied.