I've recently posted this on Twitter but I thought I'd post a longer version here.
One day this week I spent a fruitless hour trying to hunt down the location of Brilos, which according to Napier and many others was supposed to be where the skirmish at Obidos on 15 August 1808, the first British action of the Peninsular War, began. I could find no mention of it on contemporary maps or books on Portugal. Its only mention seemed to be in connection with the skirmish.
So I searched for the term on Google Books and traced it back to newspaper reports from September 1808. Realising that most papers just repeated The Gazette I searched for Brilos on the Gazette website, and found it in an edition published 3 September 1808:
I immediately recognised the letter as I'd see the original at Kew and realised that Brilos was actually a transcription error for Obidos, which is forgivable given Wellesley's handwriting. I confirmed this by checking with Gurwood's published version of the same letter. So, Napier copied The Gazette and other historians followed suit. It was only because I was trying to build the narrative of the skirmish using only primary accounts that I realised that there was no mention of Brilos in any of them.