Your friend could prank you by surreptitiously putting a tiny force field across the bowl of your toilet, just above the water.
Can they be set to expand or contract? Expanding they could act like a slow bomb; contracting they could crush whatever is inside. If they can contract hard enough, then anyone with one and some hydrogen can build themselves a fusion bomb. If they can contract really hard, a black hole generator.
In the other direction, an expanding field could be used to tear open quiescent volcanoes or undermine a city. If they can be expanded very fast ( they aren’t matter after all ) they can act as explosions. Or even as multiple explosions, like a bomb that goes off over and over.
You probably would start caring about it when somebody sued you.
ETA: Your insurance company would probably have something to say about this possibility, as well.
Could these force fields be broken somehow other than by the person with the code or key turning them off? If not, what would happen if there were a fire in a building protected by a force field? What would happen if someone locked their keys inside the force field, or forgot their code?
Charles Harness already got there a decade earlier, with THE PARADOX MEN. 
My point is that force shields as described aren’t philosophically different from concrete barriers.
But if they’re less visible, they might be less safe than concrete barriers.
They might cause a change in the types of crime we see, kind of like the increase in car alarms in South Africa led to a decrease in car theft and an increase in carjacking. We might see fewer burglaries of unoccupied houses (if they were protected by force fields) and more situations where someone knocks on the door and pulls a weapon on the person who answers the door.
I wonder if a force field surrounding your house or property would be in violation of local fence laws, which typically restrict the height of fences to four or six feet, and sometimes restrict the materials that fences can be built from. Definitely look for “spite fields”, analogous to spite fences today, and other uses of force fields to annoy neighbors. Someone might turn on a force field around their neighbor’s house to prevent the neighbor from holding a party, for instance, or annoy them by using a force field so the neighbor couldn’t get into his house.
Could sound get through a force field? Cell phone transmissions?
Assuming we don’t quite understand the technology behind them, presumably they’d produce some sort of radiation and it could be harmful to humans.
[Sir Humphrey Appleby] I foresee all sorts of unforeseen difficulties! [/Sir H.A.]
[Old lady voice]
Help! I’ve fallen and I can’t [del]get up[/del] reach my button!
[/voice]
That’s dangerously close to asking for details, Anne. You don’t really want to enourage me, of all people, to do something like that, do you?
I could see personal force fields being used to keep older people from injuring themselves in falls (assuming personal force fields were possible and would prevent injury).
If the force fields stopped things gradually enough, I’d want them on the floors in my house. No more injuries from falls, and no more dropped objects getting broken. It would be a boon for parents of small children, elderly people, and klutzes.
So . . . how is research for the force field condom going?
Things are kinda up and down at the moment.
…oooh…radiation burns…
Lieutenant, lower the shields. I’m ready to send a transmission.
That’s emission.
Force field chastity belts.
If they turn on at an inopportune moment, well… There might be a whole new resurgence of castrati singers.
Would they need to be physically attached to a source, like a laser, or could you turn one on remotely like cause a bubble to appear without blowing it?
Does it need to be round or can it be assymetric?
That could have wonderous possibilities for surgery- get to that appendix just before it burst and pull it towards the surface. Then just slice down and pop it out in the field.
Obviously IANASurgeon, as there must be all sorts of repair issues to clean up, but still, it would change the course of modern medicine.
Can you generate a motile field? Can I have a prosthetic which I can manipulate? Can I be doc ock?
Can I have a force strap-on to extend my member and add another, smaller one for the girls who are so inclined for both?
You realize that this leads to…
…Detachable Penii.
Well considering plasma windows temperatures can be a few thousand degrees at least, KY warming will have a new competitor!
The answer is, of course, “depends”. We don’t know the physical properties of such a hypothetical shield. Would it behave like a pair of same-poled magnets, exerting a repelling force relative to the square of the distance from the sheild projector? In which case I think it would be relatively safe. You couldn’t really crash into it. You’d just sort of deflect off.
If it behaved more like the instant creation of a rigid indestructable bubble, it might be a lot more dangerous.
Would it be permeable to the atmosphere or certain materials?
Is it unidirectional or does it work in both directions?
Is it dangerously hot? Or cold for that matter. Or electrically charged or radioactive? Does it radiate light? Any of these could cause problems.
Is there anything that counters it? Like reversing the polarity of a reflector dish?
Does it “fail on” or “fail off”. IOW, if I create a shield and smash the generator, does the shield go away or am I stuck with it with no way to turn it off?
If I turn it on in an enclosed space, will the shield push any obstructions out of the way, cut through them or just work around the object?
I’m sure that’s from some book but it sounds like the most asinine hypothetical design for a defensive shield imaginable. Kind of like a tank made out of some sort of material that explodes on impact.