When playing Mashed, most likely you've experienced the jump bug, one where you trip over nothing while jumping, and fell down, making you lose the race. This is especially annoying in maps where jumping is part of the main track, since there will be no other way around. For example, this picture is from the Tierra Pedra track, while fighting the helicopter.

Right-click on Mashed from the Steam Library. Right click on it, select Properties, then Local Files. Next, click on the Browse Local Files option. This should open a new window in your Windows Explorer.

Switch to the new Explorer window that just opened. Find the MASHED application in the folder, and right click on it. Then, click Properties.

Open the Compatibility menu, check the box, and run the program in compatibility mode for Windows XP (Service Pack 3). Click Apply, close the window, and launch the game.

The fix should have worked, and jumps should work normally. However, there is ONE side effect. The Steam Overlay became disabled, so you cannot open your overlay, chat, or take screenshots while running the game.

There is actually another simpler way to overcome the jumping bug. When you're about to jump, swerve your car a little to the right or left, by tapping on the turn button. Sadly, this doesn't work all the time (but it rarely fails, most of the time it works), and can mess with your handling a bit. Personally, I prefer to just swerve a bit before a jump rather than changing the compatibility mode.

Credits to Goose274 for finding the compatibility mode solution on a Discussions page. I created this guide so it can be found easier by new players. Link to discussion here.

I played Mashed on a Windows 7 PC, so I'm not really sure if this solution would work on older or newer systems. If you tried this method on a system other than Windows 7, feel free to share the info.