I wrote a series of VB macros in May this year where I
would open an excel file sometimes with a ".xls" extension
and sometimes without.

Code variations looked like this:
Workbooks.Open Filename:=("C:\test.xls")
Workbooks.Open Filename:=("C:\test")

Both versions of code worked fine for 6 months.

Then in October any that did not have a ".xls" could not
be found, even though the code was not changed in any way
nor was the software version. eg. Workbooks.Open Filename:=
("C:\test")

So I changed all of the macros to include a ".xls"

Now in December, one of my macros cannot find a file
unless the ".xls" extension is removed, and I check the
file and made sure it was a standard excel 2000 workbook.

Does anyone know why this is happening as it defies logic?