It’s never a good idea to mow your lawn when the grass is wet. For one thing, you can slip and sprain an ankle. Then there’s the worst-case scenario: Your wipeout results in a foot or hand slipping under a mower blade that's churning at 200 miles per hour. Mowing wet grass can also hinder your riding mower's ability to turn and stop. And Paul Tukey of Safe Lawns has heard of at least one instance in which a riding mower plunged off a hillside because of a saturated lawn.