Quote from the Gaia FAQ:
Why are some parallaxes less than zero?
Negative parallaxes are caused by errors in the observations. Even if a negative distance has no physical meaning, there are a certain number of stars expected to have negative parallaxes just from an error propagation perspective. The negative parallax tail is a very useful diagnostic on the quality of the astrometric solution. Further details can be found here and here.
Yes my mistake. What I meant was getting the parallax to determine the error in the parallax is not very useful. Instead, there is a parallax_error value that can be fetched from Gaia directly to calculate relative error %.
Edit: Also I should add that not all stars in a messed up cluster have a parallax less than 0. In fact, most have positive parallaxes, with an error that sometimes is even relatively small.
Edit 2:Also in real life, negative parallaxes are not real! Negative in measurements caused by the uncertainty means that the angle could be in the opposite direction.
Oh. I have a youtube channel. https://www.youtube.com/c/Phunnie