Worst case would be if you get an electrical shock from it. Provided all wiring is insulated properly, and you used a ground on the outlet, that won't happen.
If you dead-shorted somthing, it will blow a fuse or trip a breaker if those are working properly and sized properly. If you have doubts about this, find an outlet that you can verify is wired correctly and protected with a circuit breaker.
350 watts is not alot of power. At 120V thats only 2.9 amps. Most vacuums and toasters use more power then that.
There really are no warning signs of a dead short. Things will happen faster then you can think or react.
What size wire did you use for everything?
I don't see anything particually dagerous about it from an electrical stand point.
If your in doubt of your work, double and triple check all your connections.