Let me share my story.

Some time ago, as I was starting to discover the pleasure of bikes, I read a recommendation somewhere to grease stem-to-handlebar contact, in order to avoid corrosion in the future (or maybe I misinterpreted sound recommendation to grease steerer-to-stem contact). So, next time I fussed with my handlebars, I did just that.
You can start laughing now.

So, here am I, riding some new road, going down moderately steep and long incline, and discovering that pavement has large cracks perpendicular to the road, edge to edge, repeatedly, every 30ft or so.

Of course, by the time I learned about this particular marvel of local road-builders, I am going downhill full speed, and of course there are plenty of cars zooming past me (I am going about 25-30 m/h, they are going about 35-45, you get the picture).

After about a dozen of train-like cha-chungs, each one opening my eyes a little bit wider, on a next one, my bars suddenly rotated downwards, and since I was riding on the hoods, my hands almost slipped off to the front - and my eyes certainly were like 2 ping-pong balls. Applying brakes to stop the bike was kinda tricky because the harder you brake, the harder it is to hold on those downward facing hoods. I felt a bit like clown, the one that is riding a unicycle upside down, pedaling with his arms - my arse kinda high in the air, and my hands down low, holding on to the hoods with the death grip.

After stopping and shaking for some time (because of cars behind me, if I had fallen, this would be a very sad story), I pulled the handlebars back into proper position, attempted to use short allen key from emergency tool box to tighten stem (to no avail), and continued my ride very gingerly.

Lesson number 1: DO NOT grease both ends of the stem! You can insert your dirty jokes here.

Lesson number 2: as I was finishing that ride, I learned that riding on tops or in the drops applies much less torque to stem binder compared to riding on hoods; it became a second nature to me now that whenever I am riding on rough roads, I never ride on hoods, always tops or drops.