A rumble strip is a strip of corrugated pavement running alongside highways. If you go to sleep and and drift off the road, the wheels of your car hit the rumble strips and that makes the car vibrate with a loud BUR-R-R-R-R-R. A rumble strip doesn’t damage your car, but it’s an alarming event all the same, because it means that you’re about to have a major accident if you don’t do something to get back on the road.
Programs are like cars driven by notes left by an absent programmer. For testers, hearing the rumble strip means a good bug is literally around the corner.
The rumble strip heuristic in testing says that when you’re testing and you see the product do strange things (especially when it wasn’t doing those strange things just before) that could indicate a big disaster is about to happen. By “strange” I mean behaviors that you are confident the programmer did not intend. For instance, I was testing an input field once and at 23,829 characters all the text turned white. This is a strange bug, but mainly it served as a rumble strip under the tires of the absent programmer. I knew the program was leaving the highway, so I kept pushing with my tests until I got it to crash.