Back in 2011, my girlfriend was working at a catering company that announced shifts via a webpage and workers had to sign up for them. Other workers tended to pick them up very quickly, so it was hard to get too many shifts.
I wrote a quick web scraper to automatically accept any shift offered and email her.
For a couple weeks it was great, suddenly she had all the work she needed.
Then one day she woke up late to find a voicemail telling her she was fired.
Earlier that morning the script had detected a last minute job announced just an hour before start time and immediately accepted it, resulting in her not showing up to it. I had not accounted for the possibility they would announce a job so last minute, since it had never happened before.