I have a few "almosts"...
On our Honeymoon, DH and I didn't know there was some sort of race near the airport (Midway in Chicago) and the traffic was crazy - we very nearly missed our flight.
On our 3rd anniversary trip, DH left something in the car back at the hotel and took the hotel shuttle back to get it - we had at least 90 minutes before the flight (this time back to the normal Minneapolis/St Paul). WELL, the shuttle ended up taking a ton of people to the Mall of America on the way to and from the hotel so I was left at the airport panicking. The wonderful gate attendant actually held the plane for a few minutes when DH arrived and I ran across the airport to meet him and make it on time. (I thought they never did this, but this was pre 9/11). Needless to say, DH makes us check in and be at the gate at least 60 minutes before takeoff now

.
On our last trip, DS was a raging, snotty, nasty spoiled brat the night we got there and I knew if he didn't lose the attitude the trip would not be pleasant. Fortunately he did and we made it to dinner and then had our best trip ever.
But definitely my worst experience was in 2002. DH and I had a trip planned for our 5th anniversary and had booked a 2 (or 3, can't remember) br villa at Sheraton Vistana through our Interval timeshare. My mom and dad were going to watch our DS (who would be 1 at the time of trip) while we were gone. We couldn't wait! And then, when we ended up getting incredible airfare, my mom and dad decided they would come with and we would split watching DS. DH and I were beside ourselves - my parents are magnificent people and DH says how lucky he was to get great in laws (I, however, was not as lucky with my inlaws

). Showing my mom and dad WDW in 2002 was something we looked forward to - I had been there with my family in the early/mid 80's but they hadn't been back since.
Then everything changed at the end of July when my mom (my very best friend for my whole life) was killed in an accident. The trip was less than 2 1/2 months afterwards and with only DH, DS and me it was not what we had planned. We were still very much in mourning and it could have been horrible. Instead, we had a wonderful respite from the sadness while we watched our little baby boy take in everything there was to see at WDW. He wasn't old enough to "get it", but he was fascinated by the magic. And although it may sound strange to anyone who doesn't love WDW the way we do, the trip really helped us.
And every trip since has been wonderful - especially since we joined DVC and started staying on site...!