The only possibility is that the washer was too large. It should have quite a close fit to the bolt. The clearance should be about .030 per size, maximum, and the outside of the washer should be of the approximate diameter as the head of the bolt. Take a look at the washer, now, while it is installed. Can you see more of one side of the washer sticking out than the opposite side? Is the washer approximately the same size as the bolt head (maybe just slightly larger) or is it quite a bit larger?

The only other possibility is that you are a bit scared of torquing the bolt properly due to your last experience. It is not all that easy to strip the thread out of the bolt. You remember all the effort that you put in the last time. You might want to get a torque wrench and use it to be sure. The bolt should be torqued to 30 ft.-lbs. It will not leak if you reach this torque level.