They do not slide off. But here are some negative scenarios:
You have failed to keep the silicone grippers clean. Please when you wash the Thigh Guards, make sure you specifically clean the silicone grippers, if they get dirty (invisible dirt / dead skin cells), then they will fail to grip onto your legs immediately. They will grip to your legs eventually, but the first 5-10 minutes will not be fun. (Note. It takes a couple of weeks of neglect for the silicone grippers to be blanketed in dirt, but it's recommended you clean them after each use, just to make sure you'll always be fine)
You apply cream, oils, or lubricants to you thighs. Please avoid the application of these products to your thighs whenever you want to use the Thigh Guards, and make sure the effects of these creams and oils are not still prevalent when you want to use the Thigh Guards.
You live in an extremely cold climate. Full disclosure we did all of our testing in Australia, the lowest temperature we tested in was 8°C / 46°F and they worked fine, as long as you're active. Cold weather is kind of the kryptonite for silicone gripping to any moisture or sweat on your skin. Hot weather is humid and creates moisture, cold weather is dry and evaporates moisture, and this applies to your skin. So the skin will be dry and smooth and the silicone grippers may have trouble sticking to your skin. When it comes to winter, you are actually far less likely to get chafing, because the weather evaporates all of your sweat so quickly, so your skin stays dry, smooth and chafe free. One of the main contributing factors to chafing is sweat and heat, and these things are not present in winter. So you might find that you put your thigh guards away in winter, and then bring them out and use them frequently in summer.