I had four solid-fuel engines attached to the top of my ship that burned at the same time as three liquid-fuel engines at the bottom.
When the solid-fuel engines burned out, I decoupled them, only to learn that it also decoupled the still-burning liquid-fuel engines.
There was another single solid-fuel engine that I planned to ignite once the liquid-fuel engines were decoupled, which they were.
Luckily I didn't have the decoupling of the liquid-fuel engines on the same stage as the ignition of the solid-fuel.
The liquid-fuel engines pushed the solid-fuel engine, the SAS module, and the capsule, about 20000 more meters before it burned out and I ignited the solid fuel engine and flew off.
Does that make sense? :o
Here's a picture to explain it all:

The liquid-fuel engines you see are actually decoupled from the rest, but the g-force from the thrust keeps the rest of the rocket attached. The solid-fuel engine closer to the capsule could have actually been ignited at that point, but I chose not to until the liquid-fuel engines burned out.