I noticed that my Spanish teacher sometimes does not pronunce the letter "s" in some syllables. For example, she says Ih-rael instead of Israel. Or she says "mah" instead of "mas". The thing is, she does this occassionally and doesn't seem to be aware of that.
I made a small researched about it and found out that this is something called s-aspiration and it is common among native speakers. But I wonder when and where does the letter "s" may become aspirated.