There are two problems with the zoom function, at extremely large zoom factors (or small angles) :
1- Celestia becomes extremely laggy at large zoom factors, especially when I use the zoom for the first time after starting Celestia. This is systematic and easy to reproduce on my Mac, and occurs even if there's nothing visible on the front (or maybe some far away galaxies ?). Nebulae aren't activated.
The lag problem doesn't appears to be related to the nebulae, which may be desactivated.
2- While zooming, activating the nebulae will bring some object to the front, while it shouldn't be visible at all since that object is supposed to be in the back of the observer. For example, some large graticule model defined as a nebula, or even the Hubble ST model itself, may become visible on the front while the model is actually in the back of the observer. Hubble is then visible as a tiny black spot (looking like a small fly) in the center of the screen. It shouldn't be visible at all since the model is located behind me ! Reminds me the known optical illusion in a "closed universe".
I suspect both problems are related in some way.
May be a Mac specific bug (?).