A UI packed with seemingly random, disparate icons is a usability disaster. I know what happened: you started using an icon set because it looked cool, then you realized it didn't have an icon for "upload" or "download".

The UI review meeting is later on today and you'd better get an icon in there quick! So, you use and instead. Except, they look totally different to the rest of the app and users have to spend those extra few seconds working out that, yes, they are actually part of the UI too.

Don't be lazy when it comes to icons: pick a metaphor and stick with it. This may mean extra illustration effort to produce new icon elements that are in keeping with the icon style, but that effort will pay off in increased usability for the end user.