According to the OpenStreetMap Wiki, line=* should only be used for power lines. Some mappers still tend to use line=bus on bus route relations for reasons I don't know 😄.
Especially as this is no additional information if route=bus is also set, this value should not be used at all for bus routes. This is not really an error, but more a warning which should appear if this value is set.
See e.g. https://www.openstreetmap.org/relation/2078316
According to the OpenStreetMap Wiki,
line=*
should only be used for power lines. Some mappers still tend to useline=bus
on bus route relations for reasons I don't know 😄. Especially as this is no additional information ifroute=bus
is also set, this value should not be used at all for bus routes. This is not really an error, but more a warning which should appear if this value is set. See e.g. https://www.openstreetmap.org/relation/2078316