Here is the original item:
While Dr. Kinser bounced down the
stairs (A) and
strode through the hallway.
(B) A brightly colored
tie streamed over his shoulder,
(C) flapping
in his wake.
You wanted to do this:
While Dr. Kinser bounced down the stairs,
and strode through the hallway. A brightly colored tie streamed over
his shoulder, flapping in his wake.
Adding a comma after stairs is an error. Notice that you have two items (not complete sentences) joined by and—the verbs bounced and strode. This pattern does not require a comma with the and.
You might want to review the rules.