Go home Steam, you're drunk.
Or perhaps someone behind the scenes is. This appears to be the common consensus as several users logged into Steam today to find out that a majority of their library were either downloading empty updates with no size, or downloading updates that were usually slightly above 100MB but have gone over 300MB in size in some instances. In some cases, the empty updates were followed by the few hundred megabyte updates. There's a reddit thread and several threads on the Steam forums about the issue happening.
The reason for these "updates" was actually that these updates were deleting redistributables for Microsoft Visual C++ and DirectX in games that they were packaged with (the writer of the article has confirmed this himself). This is a common practice for anything downloaded over Steam that use these to function. Starting a game for the first time over Steam always has a setup process that involves checking to see if either Visual C++ or DirectX components are up to date for the game that is being played, and installing any missing components if they are not present or up to date. Several users (including the writer of this article) then proceeded to have Steam redownload any missing redistributable installer, the cause for the 100-350MB updates, though some users have reported higher sizes for some games. Users that have previously deleted these folders through some fashion (Steam Cleaner, manually, etc) have also been subject to these downloads.
While this on the surface wouldn't be a big deal, in an age of increasingly metered and capped internet connections, these updates could squander precious megabytes, even gigabytes off of affected parties.
The cause appears to be related to Valve working on repositories behind the scenes today, according to a mod responding to one of the several threads popping up across the Steam boards, though it seems the post was later updated that something indeed went wrong during this operation. If these updates are happening to you, the same mod urged you to instead restart Steam. Being opted into Steam Beta apparently does not affect the occurrence of these updates. Also important to note is that Steam Mods don't have much more information generally speaking and are not Valve employees.
As of this writing, there has been no response from Valve, or an official confirmation on the actual cause of the problem. We have reached out to Valve about the issue and will update when we have more information.
Have you experienced these issues? Do you have data maximums that may make an error like this potentially costly? Share your thoughts in the comments below!