Flight Historian tracks all my airline and charter flights . However, I’ve taken a few flights which are not logged in Flight Historian.
Non-Airline Flights
I’ve had a few flights not operated by an airline which were therefore not logged. This includes a number of general aviation flights, several helicopters, and a biplane ride.

My (now) wife and I riding in a Travel Air 4000 biplane on 9 Jun 2014.
Flights Not Leaving the Ground
I’ve boarded a few flights where we ended up deplaning before we even left the gate (usually due to a maintenance issue), or taxied around but never left the ground before returning to the gate (usually due to weather). Since the wheels never left the ground, I didn’t count those as flights in my flight log.
On one particular trip, I was trying to fly from Wichita Falls, Texas (SPS) to Dallas/Fort Worth (DFW) and then on to Dayton (DAY), but the SPS–DFW flight was cancelled. Since it was only a small 50-seat regional jet and SPS was less than a 2-hour drive from DFW, American Airlines hired a number of vans from a taxi company to drive us to DFW, but still issued us a flight number, boarding passes, and seats for that “flight.” Even though American counted it as a flight for all intents and purposes, I did not include it in my flight log, as the van’s wheels never left the ground.

Screenshot from the American Airlines app, showing our “Flight” 9289 operated by “A Bus.”