To reproduce:
  RadMessageBox.Instance.Font = new Font("Segoe UI", 10);
            foreach (Control c in RadMessageBox.Instance.Controls)
            {
                c.Font = new Font("Segoe UI", 14);
            }
            RadMessageBox.SetThemeName("TelerikMetro");

            RadMessageBox.Show("test", "test2", MessageBoxButtons.YesNoCancel,                  
            RadMessageIcon.Error, MessageBoxDefaultButton.Button2);

WORKAROUND: use smaller font size

To reproduce:

Create a RadForm and set the property as follows:

public Form1()
{
    InitializeComponent();
    this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
}

Workaround:

Set the ShowIcon property to false:

public Form1()
{
    InitializeComponent();
    this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
    this.ShowIcon = false;
}

To reproduce:
Add a RadRibbonBar, associate AcceptButton and CancelButton and subscribe to their events. Run the application press escape and the CancelButton's event should be fired. Press enter -  no results.
Workaround:
protected override bool ProcessCmdKey(ref System.Windows.Forms.Message msg, System.Windows.Forms.Keys keyData)
{
    if (keyData == System.Windows.Forms.Keys.Enter)
    {
        this.AcceptButton.PerformClick();
    }

    return base.ProcessCmdKey(ref msg, keyData);
}

Resolution: 

The issue is fixed in Telerik`s RadForm and RadRibbonForm. If you use MS Form, this is still not working.  

Workaround: use RadRibbonForm instead or set minimum and maximum size

To reproduce:

Show RadMessageBox as follows:

RadMessageBox.Show(this, "מסר כלשהו", "Caption", MessageBoxButtons.OKCancel, RadMessageIcon.Error, MessageBoxDefaultButton.Button1, RightToLeft.Yes, "מסר2 כלשהו");

Open the details button and you will see that the text is כלשהו מסר2

FIX. RadForm text flickers on change

To reproduce:
- Set the form MinimumSize to or close to the RadForm's actual size.
- Turn all windows visual effects off
- Set the theme to Office2010Silver
- Start the application

Workaround: 
- Load the theme manually in code like this:
 ThemeResolutionService.LoadPackageFile("C:\Office2010Silver.tssp")
 ThemeResolutionService.ApplicationThemeName = "Office2010Silver"

Steps to reproduce:
1. Add a RadPropertyGrid to a form and subscribe for the PropertyValidating event
2. In the event handler show a RadMessageBox

Run the project and change a property. Try to save the value and you will see an exception.

After each call to the Show method of RadMessageBox, the message box form is disposed. This causes each new call to Instance (or Show which internally calls Instance) to create a new instance of a message box form, loosing any user settings.

To workaround this issue you should dispose the Instance property of RadMessageBox after each show of message box. For example:

WORKAROUND: 
RadMessageBox.Show("Text");
RadMessageBox.Instance.Dispose();

If you are using themes, consider the following approach:
            RadMessageBox.SetThemeName("Windows7");
            RadMessageBox.Show("This is some long text that sometimes does not wrap as it should.");
            var field = typeof(RadMessageBox).GetField("radMessageBoxForm", BindingFlags.NonPublic | BindingFlags.Static);
            field.SetValue(null, null);

When RadMessageBox has not owner and it is using in form that has TopMost = true throws exception after first call of RadMessageBox.Show()

Steps to reproduce:

1.Create From with TopMost  = true

2. Add button and insert the following code in press event:
RadMessageBox.Show("Text");
3.Run application and press the button two times.

Workaround after each show of RadMessageBox add this code snippet:
RadMessageBox.Instance.Dispose();

RadMessageBox  throws ArgumentException - "A circular control reference has been made. A control cannot be owned by or parented to itself." when using with MDI forms. 

WorkAround:
RadMessageBox.Show("Text");
RadMessageBox.Instance.Dispose();

RadMessageBox - SetThemeName method is not working correctly, after dispose on owner form.

Workaround - Reset the radMessageBoxForm field via reflection. For example:

            var field = typeof(RadMessageBox).GetField("radMessageBoxForm", BindingFlags.NonPublic | BindingFlags.Static);
            field.SetValue(null, null);

            RadMessageBox.SetThemeName("Office2007Black");

Default button focus is not correct and additional tab press is needed.

When HTML-formatted text is displayed in RadMessageBox, it is cut off on the right.

When you are navigating repeatedly between the buttons in RadMessageBox using the arrow keys, at one of the steps none of the buttons is focused.

RadMessageBox - Buttons are not arranged correctly in RTL

FIX. RadRibbonForm - setting the FormBorderStyle property does not take effect

When a user double-clicks the OK button at design-time, an empty event handler is created.
However, this handler needs the WithEvents keyword in the Designer of the form.

If the theme of your Windows 7 is 'Windows 7 Basic' and you have a RadForm as an mdi child., when you maximize this form, you will notice that there is a gap of several pixels between the parent form title bar and the client part of the child form. This can be worked around as shown below:

Public Class MDIParent
    Public Sub New()
        InitializeComponent()
    End Sub

    Dim shouldGetSavedSize As Boolean = False
    Dim suspendResize As Boolean = False
    Dim formSize As Drawing.Size = Size.Empty

    Private Sub RadButton1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RadButton1.Click
        Dim form As RadForm = New RadForm
        form.MdiParent = Me
        form.Show()
        formSize = form.Size
        AddHandler form.Resize, AddressOf Form_Resize
        AddHandler form.SizeChanged, AddressOf Form_SizeChanged
    End Sub

    Private Sub Form_SizeChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim form As RadForm = DirectCast(sender, RadForm)
        If Not form.WindowState = FormWindowState.Maximized Then
            If suspendResize Then
                Return
            End If
            formSize = form.Size
        End If
    End Sub

    Private Sub Form_Resize(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim form As RadForm = DirectCast(sender, RadForm)
        If form.WindowState = FormWindowState.Maximized Then
            shouldGetSavedSize = True
            form.FormBorderStyle = Windows.Forms.FormBorderStyle.None
        Else
            If shouldGetSavedSize Then
                shouldGetSavedSize = False
                suspendResize = True
                form.FormBorderStyle = FormBorderStyle.Sizable
                form.Size = formSize
                suspendResize = False
            End If
        End If
    End Sub
End Class