You should have signed a MOU or contract and indicated an after-care service. I myself as a freelance developer will normally take care all bugs in my code if there are any and if proved as my fault within one year after the hand-over. The bug you showed here is the programmer's fault obviously.
CByte() can handle number up to 255 only, you'll get Overflow error if the number is higher than this.
Dim varLookup As Variant, lngMax As Long
varLookup = DMax("Seq", "ECNPartstbl", "[ECNBCNVIP ID]= " &
If Not IsNull(varLookup) Then
lngMax = Val(varLookup) + 1
lngMax = 1
Me.Seq = lngMax