My macbook pro hard drive is stupid?

[subsistence5]

Say i have 30 Gigabytes free on my hard drive. Then i start a download that is 5 gigabytes. Before it really downloads much (like 50 megabytes) i cancel the download and delete the already downloaded data. After doing that, i go to check my hard drive space and theres only 25 gigabytes. I am running Lion. I really need help because I don't have much stuff on my computer and this bug/glitch is tricking my hd to think theres is space being taken up. The hd was at one point full and i had to delete some of my own files. I was wondering if there is any way i could access my hard drive and find files that i cannot see or perform maintenance or something to clear unknown files. I really need help as soon as possible because I need this computer for other things and the low storage space thing is a problem.