It’s definitely a problem. You should never have any destructive or modifying actions of any sort done from a GET request. In addition to browser prefetching, users might share a link without the receiving user realizing they are taking an action as soon as they click the link, which can be a source of security issues.

That being said, I too have seen things like this on the Internet. I would assume Google has some sort of heuristic to determine whether or not they can safely prefetch a link, but who really knows? You should absolutely avoid making this mistake in your own code.