The scariest failure was loss of throttle while in a current, waiting for a drawbridge to open. Thankfully, I was waiting alone. Any attempt to throttle-up would result in the engine sputtering and threatening to stall. All I could do was slip it into gear and idle, hoping that it was enough to go against the current that was drifting me toward the bridge supports. After about a minute of trying to get away from the bridge at idle, I saw I was loosing the battle and drifting under one of the side arches. I was single-handing that day and didn't have a working auto-pilot to steer, so I ran to and from the helm and the mast-boom, antennas, dinghy crane, and anything else with a clearance issue. All I could hope for is an idle controlled drift, backwards under the bridge. I had my bow thruster to help with alignment between the arches, knowing that I might have enough clearance in the center, but I had to stay away from the curves of the arch on each side. I cleared the first arch with just a nick on the starboard roof when the boat rocked a bit, but it pulled the boat a bit sideways and now I had to align the boat again before it got caught on the next arch. Full rudder took care of aligning the stern, but using the bow thruster partially negated the action of the rudder so I couldn't use it until my pilothouse roof was already on the arch. Using the arch as a spring, a touch of the bow thruster helped the roof roll around it and clear the rest. Once she cleared the bridge, she was already in a turn and there was no oncoming traffic, so I just used the thruster to straighten it out and the idle speed with the current was enough to get me to the closest anchorage possibility, but by the time I arrived, the throttle problem (probably air in the fuel system) had dissipated. I anchored, changed the filters, tightened fittings, etc., but never knew what happened. No problem since.
I never saw the bridge-tenders face, but i'd guess he'd probably seen stranger things than that anyway.