What's the oldest unresolved, and significant software bug you know of?

If you open an Excel file on different platforms (Windows vs. Mac), date/time values will be four years off if the value was entered on the other platform.

Selects via index occasionally does not find data when using serializable transaction isolation on Oracle, since about 25 years (see https://stackoverflow.com/questions/11826368/oracle-select-i... )

The Windows Blue Screen of Death. I don't think Microsoft will ever fix it.