There is a similar user error that occurs sometimes where players double right-click or have their hands stuck in the ready position and it produces the same result. This issue can easily be tackled by double right-clicking again, or using the spacebar key.
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I would like to note that the issue described above is ENTIRELY UNRELATED to this one. The result is the same --- the cause is not. The fix also does not work for this. This is 100% legitimately a bug.
It has been reported several times. I've seen in posted since as early as January 2013 on Steam forums, and I've seen a few reports on the bug tracker here as well. For some users the problem fixes itself months later without any explanation. Most likely a hotfix by the devs. However, this problem still occurs for many others.
From what I've gathered this seems to happen more often with users that have low PC specs.