People usually mow lawns whenever the grass gets or begins to get too tall. They'll even mow the grass in the middle of December if they live where the weather is mild enough that the grass will continue to grow.
Where there is grass there is often a neighbor that doesn't have a lawn mower and continuously wants to borrow one. That's not such a bad thing to help a neighbor out by letting them borrow yours every once in awhile. It's not bad until the neighbor becomes a nuisance or they have damaged the mower.
A neighbor may damage a mower in all sorts of ways.
1) By accidentally dropping something on it.
2} By spilling something on or in it.
Lastly, by where they drive the mower and what they cut.
If you happen to be outside and see what the neighbor is driving the loaned mower over you may never let them borrow it again. Things the neighbor may drive over are the driveways, the sidewalk, small brush and trash. Don't forget if you have a gas mower they may use up all the gas and never fill it up.
When a neighbor drives the mower over a sidewalk with a few rocks to driveways that are made up of rocks not only can they get hit by flying rocks but also those rocks can dull blades and cause destruction to the motor. The damage may be so bad that you have to buy a new mower. If the flying rocks hurt the neighbor they may try to hold you responsible for a faulty mower.
When a neighbour drives over small brush the same can happen. They can dull blades and cause destruction of the motor from flying sticks and they could get hurt such as a flying stick poking them in the eye.
Mowing over trash can cause dull blades, destruction of motor and a hurt neighbour.
Before loaning out your mower to a neighbour take into consideration how long they have been at their place of residence. Do they own or rent the property? Renters who frequently move probably wouldn't care what they did to the mower whereas someone who owns the property would be more apt to take better care of it that is unless they've been living at that address for years and continuously want to borrow a mower.
Also check out what the neighbour's lawn or property looks like. Does it contain a lot of large and small sticks? Is it trashy and contain stuff that belongs in a Dumpster?
One last thing to consider is how long have you known the neighbour? Are you friends? Would they take good care of your things if you let them borrow them and return what they borrowed in the same or better condition?