Also, always make sure you are using the correct "nationality" of bolt, too.

I know from certain old British bikes that the dropout eyelets were sometimes tapped for British threads (BSC or even Whitworth). This means that a standard Metric bolt might begin to screw in but will soon tighten excessively and then freeze up to the point of failure - just as you experienced... and even with surprisingly low torque.

If a dropout or frame boss appears to be excessively rusted, you can still commonly "chase" the threads by simply installing the right bolt. But it's good to have a standard metric tap on hand to clean it out first. A 5mm metric tap (and 3mm for Campy dropout adjuster screws) are the most useful to have for bicycle applications and are a VERY inexpensive form of "insurance". Tapping existing bike threads usually requires very little force, so you don't even need a fancy tap handle or a complete die & tap kit.

And, it's a good idea to also lubricate the dropout with an anti-seize lubricant which not only eases the bolt in but tends to also inhibit further rusting. A small tube is usually available at any auto parts store, costs very little, and will suit your bike needs for years... it should be considered a "Must Have" lubricant for use with any alloy quill stems and seatposts, anyway.