$core_v2_forumReply.GetForumReplyPageIndex/forumReplies.GetPageIndex IncludeThreadStarter = true doesn't work

First, you will need to share how you are using it, as in the code you are trying to get to work.

Second, I recommend looking at upgrading to at least the latest version of 10.1.  At this time normal maintenance is not being done on 10.0

So without seeing your data, are you sure that is not correct?  That the reply in question is not index 0?  What is it you are trying to accomplish?

And I mean on page index 0

when i remove the parameter IncludeThreadStarter, the page index is returned correctly, when i set it to true, the page index is returned as 0.

I guess my question really comes down to what are you trying to do?   As in your overall goal..and while it doesn't speak to whether its functional, this is an older API

one of the places i'm using is to generate a url for a forum post in the activity stream, in order that when you click on the link it takes you to that post, on the correct page.

Does the reply.Url not giving you a permalink that works?   If you click on the date portion of the reply you can see the link thats created

that only worked if the reply was on the first page

I can look at the API, but are you trying to rebuild the thread UI or story UI?  I only ask because out of the box, the thread story stream with replies and current UI already does what it sounds like you are trying to accomplish(in 10.1, more than likely 10.0)

we have customised thread story type to change the formatting. We are using the Telligent v10 velocity widget, with only a change to the url on the the date.

You should actually be seeing an error in your exceptions, a SqlException to be exact.

You can try this, though here is the disclaimer:   

We do not support modifying any part of the database or its procedures, the change I am suggesting you do at your own risk.

With that disclaimer out of the way, I only suggest this because you are on 10.0 and would never get a fix for this until at least a future 10.1.

If you modify the [te_Forum_ThreadReply_GetPageIndex] (back it up first), find this line:

IF @IncludeThreadStarter = 1 SET @sql = @sql + N'SELECT -1 AS [ThreadReplyId], T.DateCreated AS [ThreadReplyDateUtc], T.UserId FROM dbo.te_Forum_Threads AS T WHERE T.ThreadId = @ThreadId UNION ALL '

Change T.DateCreated  to T.DateCreatedUtc and execute.   See if that fixes the issue.

In the meantime I have filed this: