It's not sloppy. It's an error.
It is, but it doesn't hit you in the face quite as much as failing to capitalise a country ;)
Having said that, and it took me a while to find where the posts had been moved from, it seems a bit harsh that the guy is being berated for not capitalising the word American. He might just have written his post quickly and not thought about it too much. Plenty of times I've capitalised words that shouldn't have been or forgotten to altogether! I usually realise later when it hits me in the face and it's too late to edit, but this is just an internet forum...
'errors' such as beginning a sentence with 'and'.
Is it an error though? I had it bashed through my skull at school to never begin a sentence with 'and' or 'but', or any other conjunction come to think of it, but conjunctions have been used at the start of sentences in literature way before it got popular to say it was a huge grammar faux pas. I've often started sentences using 'and' on this forum.
And even Mary could assure her family that she had no disinclination for it. [Jane Austen]
So we had quite a long talk when you were looking at the ruins, and he told me all about his life, and his struggles, and how fearfully hard it had been. [Virginia Woolf]