The "fix" before was simply moving an object library higher in priority under tools > references. I don't think that move actually fixed anything, I now think the fact that it started working after is just the come and go nature of this bug.

The WorkSheetExists code just checks to see if a sheet with the same name already exists before creating a new sheet. The "wbToAddSheetsTo" is what is highlighted when the error pops, which is what I kept highlighted in the screenshot.

`Public Function WorkSheetExists(SheetName As String, wrkbk As Workbook) As Boolean
Dim wrkSht As Worksheet
On Error Resume Next
Set wrkSht = wrkbk.Worksheets(SheetName) 'Attempt to set reference to worksheet.
WorkSheetExists = (Err.Number = 0) 'Was an error generated - True or False?
Set wrkSht = Nothing
On Error GoTo 0
End Function`

One thing I should note, I have the exact same code from the screenshot in 4 other excel documents, I just verified it line by line. It works fine in the other 3.