Isn't it their job to check errors?
Not really. After 9pm on election night there were none left and there were only a few drifting around today. We were checking LC papers today and we somehow ended up with two of them hovering around us - at least until we checked the votes applicable to one's area (he was a senator) at which point he had to leave due to conflict of interest - whereas speedy table had no scrutineers with them. Scrutineers are only interested in whether the votes that should be going their party's way are being properly assigned.
I am shocked that a Senator would have any conflicts of interest!
The job of scrutineers is to make sure that any votes that are being put on the piles of other parties (a) are formal and (b) aren't yours. (If it's a potentially complex preference situation, being able to track preference flows is handy, too). It has been known to try to convince inexperienced scrutineers from other parties that they should be watching their own pile....
My all-time favourite scrutineering moment was at the 1999 Victorian state election when my opposite number, a Liberal MP, successfully challenged a Greens vote whose preferences would have ended up with the Liberals - I found it hard to keep a straight face at the time, and even more so three years later when his political career (he was shadow Treasurer by then) came to a sudden and embarrassing halt when it turned out he wasn't eligible to stand because he wasn't enrolled to vote.
Of course, you get to see some of the things that informal voters write on their ballot papers. Most are unprintable, but I did appreciate the person at the last Federal election who did a write-in vote for Donald Trump, in English and Russian.
There was a particular drawing that was preferred by those with an artistic flair (or lack thereof). One voter quoted a UN standard pertaining to fluoride in the water before voting for the Fluoride Free party. Most others were either profanities, opinions on Pauline Hanson or a combination of the two.
We had one where someone had written "Have a lovely day!" which was nice.
There was a story from NSW a few years ago where somebody had written 'DICKHEADS' across their ballot paper, but the I had landed in a box so it was deemed a valid vote (NSW has optional preferential voting so a mark in one square is enough).
I was trying to decide if someone's "EAT A DICK" comment in the 7 ballot squares formed a valid vote. The second A was between two boxes so theoretically all the other letters were unique and we could have taken the initial A as the first preference. In the end I discarded it as I could see the intention of the voter was clear.