Reproducible with Unity 4.6.2p1, 4.6.1f1. Visual Studio Update 5 (2013.5 CTP 2);

Repro steps:
1. open project "Microsoft Ads Plugin 2.zip";
2. build it to WSA platform;
3. open generated solution and add reference to "Microsoft Advertising SDK for Windows 8.1";
4. build and run. Press any "Display ad banner" button and observe an exception;

Exception in Visual Studio:

First-chance exception at 0x770C2F71 in Template.exe: Microsoft C++ exception: Platform::InvalidArgumentException ^ at memory location 0x059EF138. HRESULT:0x80070057 The parameter is incorrect.

WinRT information: get_left can only be called from the main thread.

Constructors and field initializers will be executed from the loading thread when loading a scene.

Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

Stack trace:
[External Code]
UnityPlayer.dll!100dc47f()
UnityPlayer.dll!1065f29a()
UnityEngineDelegates.dll!13a33f69()
[External Code]

If there is a handler for this exception, the program may be safely continued.

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Resolution for this case is:
"Plugin bug. Plugin internally executes code on UI thread, which then calls Unity API, that can only be called from App thread."