On my first solo overseas trip (US around easter 2005), I booked each flight separately as I confirmed dates. Some flights were done online, others at a travel agent.
I had Southwest MSP-MDW-BWI, then a bus up to State Collage, PA (spent a few days with friends), followed by a bus back to BWI and a flight to BOS.
I had selected the flight I wanted online, but it wasn't possible to book online, so went to a travel agent.
Told them what was happening and that I needed a BWI-BOS flight at a set time/date, and showed them the bus schedule.
Instead of booking the late afternoon AirTran flight like I'd asked, the agent booked the early morning flight.
It was all correct on the paperwork sighted at the travel agents office, but was wrong on the email I had when I got home.
I quickly replied and got it fixed before any tickets had been processed.
Had an unexpected stop over in Singapore a few years ago after an onward connection had been mistakenly booked for the following day. Sat in the SIN arrivals hall trying to find a hotel for the night.

The ticket had free changes, subject to availability. There weren't any seats available in the required fare classes, so couldn't change it. The agent looked in to it, found that it was their error and discounted the cost of that hotel off my next trip.


(one of the best travel agents I've dealt with, even with that error).