Answer:
When the person writing the sentence doesn't use any sort of punctuation in the sentence at all.
Step-by-step explanation:
Run-on sentences, also known as fused sentences, occur when two complete sentences are squashed together without using a coordinating conjunction or proper punctuation, such as a period or a semicolon. Run-on sentences can be short or long. A long sentence isn't necessarily a run-on sentence.
-Nix