Seconded. I have a family of ~30 ton lifters with liquid fuel tanks mounted on top of the SRBs that feed into the central tank. MechJeb shuts the main engines down about 30 seconds into the ascent, and restarts them after the SRBs burn out. As a result, the 6-booster and 8-booster variants still have fuel in the booster-top drop tanks when the SRBs burn out, bringing the TWR below unity until they are manually staged away. I considered using <SolidFuel> = 0 but realized that other solid-fueled motors in the payload (e.g. launch escape systems, the Kosmos VA deorbit motor, my Duna lander's deobit and braking pack, etc.) would interfere with this.