Are there any kind of contaminants or other reasons that might be dangerous? I'd assume that as far as microbes/bacteria goes, eating off the ground on the moon is much safer than it would be on earth.
Lunar regolith/soil/dust is probably rather nasty stuff to ingest.
Where the dust on earth has been whirled about in air currents and rounded off, lunar dust is quite sharp and abrasive. It's similar to the difference between pebbles that have been rounded by water in rivers and coastlines, and regular uneroded rocks. This is true even for the finest of particles.
If you scroll down to the 'harmful effects' section of the linked wiki page, it details some of the problems if you breathe it in (effects similar to silicosis). While your digestive tract is largely a little sturdier than your lungs when it comes to ingesting stuff you shouldn't, I doubt eating significant quantities of what amounts to glass dust would do you much good.
Edit: turns out it's even nastier!
As @TomášZato mentions in the comments, lunar dust also has a significant amount of unreacted molecules and compounds in it. On earth, these have usually reacted a long time ago and are now inert. Needless to say, ingesting particles of reactive matter is also not recommended.
Brush it off, eat it.
Of course there are trace amounts of elements present that are potentially harmful, but the bulk is made up of benign stuff: 50% SiO2, 15% Al2O3, 10% CaO, 10% MgO, 5% TiO2 and 5-15% iron. There will be no microorganisms, no parasites, no viruses, no higher organic poisons. there will be mostly sand-like stuff.
About the trace amounts of less cheerful stuff: Consider how much regolith will remain on your dropped sand(ha!)wich - One gram would be much even if you rolled it around in the stuff. Say 1/1000th of that is Chromium (it is not), and pretend all of that then proceeds to villainly oxidise into hexavalent Chromium (the deadly stuff) instead of trivalent Chromium (the vitally important stuff). LD50 for hexavalent Chromium is 50-150 mg/kg ... And you just now ingested .01 mg/kg (if you weigh 100kg). You are safe (but don't make a habit out of it!)
The sharp edges of the stuff will not be an issue either: very large shards of freshly broken glass pose a hazard to your digestive tract, but anything on the scale of 'stuff clinging to your sandwich' does not. If you bite off a shard off a glass of water (as children sometimes do) many emergency personnel will only intervene by making them eat some bread afterwards. It all gets buffered by the slime. http://www.abc.net.au/science/articles/2007/01/18/1828923.htm
Of course it's not Best Practices to eat stuff from the ground, but a little regolith won't hurt.
It'll break your teeth. The moon is often very cold or very hot on the surface. The sandwich has potentially been flash-frozen or is now burnt depending on where it dropped.