Copy and Pasting the Cell Values from one sheet to another Sheet

ShazSh 

The two parts of the code interfere with each other because they write to the same row.

The first time you run the code, the first part of the code writes a value to column B in row 4 of Sheet3. This changes the last used column to B, so the second part of the code uses a different column than you expect: column O instead of column L (2+13 = 15).

When you run the code again, the last column is O, so the first part writes to column P, and the second part of the code writes to column AC (16+13 = 29).

Here is an improved version:

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim sht2 As Worksheet
    Dim sht3 As Worksheet
    Dim col As Long
    If Target.Address = "$B$6" Then
        Set sht2 = Sheets("Sheet2")
        Set sht3 = Sheets("Sheet3")
        'For 1st Table
        col = sht3.Cells(4, 10).End(xlToLeft).Column + 1
        If col = 10 Then
            MsgBox "You have used up all 8 positions for table 1!", vbExclamation
        Else
            If col = 3 Or col = 8 Then
                sht3.Cells(4, col).Value = sht2.Cells(4, 17).Value
            Else
                sht3.Cells(4, col).Value = sht2.Cells(4, 2).Value
            End If
        End If
        'For 2nd Table
        col = Application.Max(sht3.Cells(4, 20).End(xlToLeft).Column + 1, 12)
        If col = 20 Then
            MsgBox "You have used up all 8 positions for table 2!", vbExclamation
        Else
            If col = 13 Or col = 18 Then
                sht3.Cells(4, col).Value = sht2.Cells(4, 21).Value
            Else
                sht3.Cells(4, col).Value = sht2.Cells(4, 6).Value
            End If
        End If
    End If
End Sub