TMair, the Zabala you have that had a sear that was only partly engaging due to the sear tail being in contact with the trigger blade had probably been that way ever since it was built. But it took years of use and wear for it to finally reach a point where the jar from firing the first barrel caused it to slip out of engagement. The shrinking of wood that skeetx mentioned, or improperly tightening screws can cause these types of issues too.
These kinds of problems should be evaluated by a good trigger man if you aren't very familiar with trigger problems. A gun that doubles easily upon firing may also be prone to accidental discharge if you fall or drop it. The problem is sometimes as simple and obvious as an accumulation of dirt and crud in a sear notch that prevents complete engagement. But sometimes, the problem is a tiny bit of wear that can be very difficult to spot to the untrained eye. The sear angles might have been stoned incorrectly at some point by someone, or even from the time of manufacture. There was a thread here in the past few weeks about adjusting trigger pulls that addressed many of these issues. Some Spanish guns had a reputation for improperly hardened parts, and depending upon how frequently the gun is used, it could take many years for a problem like doubling to rear its' head.
