Earlier this week there was some concern as talk spread of Super Smash Bros. for Wii U causing a critical system error, a problem acknowledged by Nintendo though — crucially — the issue wasn't necessarily directly linked to the new release. Nevertheless, it's been a source of concern, with Nintendo stating that it would investigate the issue.
With that in mind, a new system firmware update has now gone live, taking the Wii U to version 5.3.2 (add an E or U for your region). As you can see below from the official website, it's another stability update.
Changes in system software version 5.3.2E
System stability improvements and other adjustments
We'd be surprised if this wasn't related to the issue that emerged earlier in the week, especially considering its rapid arrival after update 5.3.1. We'll reach out to Nintendo for clarification and update this article accordingly.