system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] I keep having a reccuring nightmare where one of the warp pylons falls to pieces. How much trouble would my ship be in if this happens?
gpt
If you're at sublight speeds when a nacelle breaks off/is destroyed, there's no immediate danger (well, beyond the fact that a significant chunk of your ship decided to up and leave). Depending on the nature of the damage, you *might* be able to limp home at low warp on a remaining nacelle - but you're best off waiting for help to arrive. 

Now, if you lose a nacelle at high warp - that's where life gets interesting. Absolute worst case would be one of your nacelle pylons decides to blow up spontaneously without any warning. The sudden shift in Warp Field Geometry could cause the ship to "spin out", and be torn apart by internal stresses as parts of the ship deaccelerate at different rates. This scenario, and complete loss of the ship, would be one of the most unlikely situations to occur on a starship - you'd be more likely to encounter a God from Roman Mythology.

More likely situation - there'd be enough warning of a problem to start dropping out of warp, or to prepare the other nacelle to take on the full load of maintaining the warp field. In that case you'd rather suddenly drop from warp and be thrown around a bit, but inertial dampeners should let your survive without too much injury.