It's more appealing to be presented with a full bottle of any restaurant condiment than it is to get one that looks used. Of course, restaurants don't throw away the condiments if the bottle isn't empty. They do what is called "marrying," pouring from the emptiest bottle into the fullest bottle so nothing but a full bottle ever goes out to the table. Granted, sometimes this is done with a huge bag of ketchup mounted on the wall in the service area and not from bottle to bottle, but the point is, it's likely some of the ketchup or other condiment has been around for a pretty long time, just getting topped off over and over again until it starts to bubble and then, one fateful day, a customer lifts the cap and rotten ketchup erupts like projectile vomit. (Sometimes, it's a waiter that gets hit, but in most cases I've observed it's the customer.) That's when you know it's time to throw that bottle away.
The worst condiment to marry is mustard. It's thick and especially difficult to get to drain, (though I actually did work in one restaurant where the manager made us marry those tiny bottles of Tabasco. We had to use toothpicks just to make it drain and we usually ended up hiding the half full bottles in the bottom of the trash instead of marrying them. It's not like we had four hours to do our sidework.) With ketchups, you can set them upside down in the service area and most of the sauce will eventually drain to the head of the bottle, but mustard usually requires a little more force. The way I learned to pack the mustard to the end of the bottle was to do a kind of wind up, using centrifugal force with the cap end pointed forward, swinging my arm in full rotation four or five times.
I was working in a small cafe one night, finishing up my sidework, when I attempted this wind up move on one of the tall skinny Heinz mustard bottles with the metal cap. The kind of metal cap that doesn't secure very well. I spent two hours trying to scrub the perfectly straight yellow line I created running up one painted white wall, across the ceiling and down the wall behind me and never did get the stain out of the paint. After that, I made sure I held my finger over the cap. Or sometimes, I just threw the bottle away.