Everyone has slip ups and sometime's tells a few lies or breaks someones trust. It happens to everyone, and it's totally normal. Astrologists believe that some traits from your sign may make you less trustworthy than other signs.
This does not mean everyone who are these signs are 100% untrustworthy or a horrible person, it just means that some traits they have may lead to being untrustworthy, but again everyone is different and it is very possible you know a person of one of these signs can be trusted.
From my experience, these tend to fit perfectly...
You probably saw this one coming. We all now know that a Sagittarius is the type of person that craves freedom and doesn't like to stay in one place for long. These tendencies cause a Sagittarius to sometimes be untrustworthy or unfaithful.
It's not that they do it on purpose or to hurt you, but sometimes they get caught up in something and forget the rest of the world exists.
They are constantly on the hunt for new things that make them feel alive and this can make for some issues.
A Sagittarius isn't evil, (always.) They just need to often take a step back and look at how their actions could affect the people around them.
A Leo is often the star of the show. Their awesome personality makes drawing people to them incredibly easily. They love any kind of attention, and they usually receive it.
The problem here is that a Leo may do anything in their power to get people to notice them, or get attention if they feel like people aren't focusing on them enough.
This could lead to them accidentally sharing other people's secrets, or even cheating.
This doesn't go for every Leo, it totally depends on the amount of affection they need and how they deal with that need. Like a Sagittarius, a Leo just needs to take a step back and re-evaluate the situation.
And last but definitely not least...
A Gemini can often be thought of as untrustworthy, because they are very complex people. They have very curious personalities that tend to get them in trouble.
They can be untrustworthy because they often lie to get their way. They don't do it on purpose, but it's often a reflex for them.
A Gemini doesn't always have to lie though. Lying for them is like building a wall to keep people out. Once people around them begin to open up, and share more, a Gemini will tend to work on being more open, so that they seem less sneaky.
Each of these signs, like every other sign, has weaknesses. Whether they decide to recognize these weaknesses and work on them, is up to them. If you know someone that is one of these signs that is incredibly trustworthy, it is probably because they have realized their weakness, worked on it, and moved forward.