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Occasionally, NuGet leaves a ConHost.exe process running that causes MSBuild lines to wrap.
This is a really odd one, but basically, if I use NuGet (either the interactive NuGet window, or indirectly through the MSBuild task) sometimes the conhost.exe process that gets spawned will stick around, and cause all future builds to word wrap the MSBuild output at 80 characters.
Update: apparently NuGet checking to see if NuGet has an update will also cause the issue.