This is an issue with C# and tabs instead of spaces when attempting to hit the metadata of an external assembly. It may be related the inferred position of the insertion point.

Others have documented this

There are a couple of Connect tickets here and here and a Github issue on this. There is also a discussion here.

Visual Studio 2015 Update 1

This issue is addressed in Update 1 so please install!

Poor workaround for RTM

The options dialog remembers the last page and remains there on subsequent opens. I have a keyboard shortcut to open the options pane quickly (Alt + o). I am temporarily changing to spaces, going to definition and then reverting to tabs before making any code changes. This workflow isn't pretty but neither were the 3.0 Nuget issues in VS2015 either (Nuget 3 has improved to date).

VS 2015 Update 1 should fix this problem.

I have made an extension which simply enables/disables "Keep tabs" on each call of "GoToDefinition" command, and seems to work: GoToDefinition Fix

I found that; if you set Keep Tab under Option -> Text Editor -> All Languages -> Tabs, and it's the same under C#, the F12 and Alt + F12 Works just fine.