We've all made the mistake of getting units mixed up. For example, putting feet into an equation that requires inches. Or mixing up Rankine and Kelvin when inputting temperature. It's super easy to make this mistake, but it can cause you a lot of headaches.
A real common one for petroleum engineers is to mix up PSIA and PSIG. It's only a difference of 14.7 psi, but it can mess up your calculations. If you see something that doesn't quite add up (literally or figuratively), go back and check your units. Especially is your pressure estimates are mysteriously about 15 psi off. I'm speaking from experience on this particular one.
If you want to see an example of how mixing up units can go very wrong, look no further than NASA's Mars Climate mission of 1999. If you've not heard of this, check out this 1 minute video:
The moral of this story is to check your units. In fact, you should probably double check them just in case.