The problem occurs from 2014 Q2.  It should be resolved with lib version 2014.3.1319.

Version 2016.3.1024.45 (from nuget packages, if that matters)
Adding a new row with the "Add new row" button and then clicking the row indicator results in an extra row. Demo: http://recordit.co/Nn8y2z2YpI

Available in the R1 2017 SP1 Release.

Just run the attached project and scroll horizontally to the very end several times (at least two times).
I tried disabling the column virtualization but this did not help.

This issue will be resolved with Q2 2015. 
A new virtualization mechanism for ColumnGroups will be introduced with Q2 2015. A property EnableColumnGroupsVirtualization is exposed with default value True for enabled virtualization. Setting it to False will enable the old logic. 

When you define a style targeted at GridViewCell with a color without transparency, horizontal grid lines are hidden.

Border is not correct when FilterRow is on and Column header is wrapped. Sample and Screenshot attached.

There are cells exported for the group rows, but they are empty. If you define ColumnGroupDescriptors instead GroupDescriptors, then the exported data is the correct one.

This issue manifests when RadGridView loaded some columns first (which are working) and then add more columns. The additional columns cannot paste values in their cells.

To work this around, mark the new columns as auto generated and call one of the internal methods.

newColumn.IsAutoGenerated = true;
this.gridView.Columns.Add(newColumn);

var collectionViewPropInfo = this.gridView.Items.GetType().GetProperty("CollectionView", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
var qcv = (QueryableCollectionView)collectionViewPropInfo.GetValue(this.gridView.Items);
var methodInfo = qcv.GetType().GetMethod("OnElementTypeChanged", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
methodInfo.Invoke(qcv, new object[0]);

Columns data gets overlapped when columns are hidden and shown again

The vertical scrollbar of RadGridView disappears in some cases when the last parent item is expanded and then collapsed. The issue reproduces only with the Flat GroupRenderMode and when UseLayoutRounding property is set to True.

The workaround is to avoid setting the UseLayoutRounding property to True.

IsReorderable of a column is not respected when ReorderColumnsMode is different than the default one

Fixed with LIB version 2014.1.317.

Change AlternateRowBackground (runtime) has only effects after scrolling in grid or after rebinding it.

When KeyboardNavigation.TabNavigation is set to Once, RadGridView does not lose the focus after the second time it's focused.