We were amused to find that one of the ingredients in our salad was dandelion greens. We wondered why we bother trying to keep them out of our garden, and decided that we grow more salad greens in our lawn than the garden. Sometimes liquid herbicides end up on plants they weren't intended to reach.
Winds can blow spray off target, and even a slight breeze in hot weather can cause vapors to drift.
Dry weed control products are less likely to end up in the wrong places, but in order to work they need to stick to the weeds. That means the lawn should be wet when they're applied, but shouldn't get any rain or irrigation for a day or two afterward. Most dry lawn herbicides are combined with fertilizer and that would be best to get off the grass after a day or so. I've already had a visitor bring in what she thought might be casualties of the weed wars. The margins of the leaves on some hosta were brown and the shape of the leaves had been distorted.