Many years ago I too was a BT retail customer, before the wonders of LLU, and it was always the poor admin that made life problematic. Basic billing errors, a failure to remember requests or agreements that had previously been reached.. etc. I've never once experienced any of those problems in all the ISPs I've passed through since, even the bigger ones.
On one occasion they added a premium service to our bill that we've never requested. It took a month before they agreed to remove it, which was harder back then as the rules governing consumer protection were wafer thin. As per Tom's comment, these days I always make sure to record and log any agreements.. just in case.
Expecting the worst and preparing for that helps you to avoid being cheated in the future because somebody, somewhere, will always try to rip you off.