oh - in case it helps, here is the code currently on my tab with the
validations. I don't know much about code, but only tinkered with it a
little. It was working fine until just today. Anyone see anything in this
that would make it unstable?

here's the code (from 'view code' after right-clicking the sheet tab):

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
If Target.Count 1 Then GoTo exitHandler

On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler

If rngDV Is Nothing Then GoTo exitHandler

If Intersect(Target, rngDV) Is Nothing Then
'do nothing
Else
Application.EnableEvents = False
newVal = Target.Value
Application.Undo
oldVal = Target.Value
Target.Value = newVal
If Target.Column = 8 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 9 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 11 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 12 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 13 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 14 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 15 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
If Target.Column = 16 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ";" & newVal
End If
End If
End If
End If

exitHandler:
Application.EnableEvents = True
End Sub