For example, in one book I read that Japan Air Lines' first aircraft was a DC-4, but the JAL website plus a couple of other resources indicate it was a Martin 202.
The DC-4 was the first actual Japan Air Lines aircraft, secondhand from USAF. However, before this they had leased several Martin 2-0-2 run for them by US carrier Transocean, who in turn had leased them from Northwest, to operate their first services. So really, either answer is correct depending on the detail of the question.
For further example, the "last BEA Comet service" was 31 October 1971, G-APMA Malaga to London, and appropriately celebrated on arrival. But the BEA Comet fleet was then mostly transferred on to holiday flight subsidiary BEA Airtours at Gatwick, and in the winter they quite often came back to Heathrow and stood in on BEA schedules when there was an aircraft shortage. Same livery, just with the word "Airtours" placed after the BEA logo. They turned up in 1972 and into 1973 doing this. So which date does one give for a "last service" ?