Using web graphics on printed material.
With many young designers coming from a predominantly web design background the transfer over from web design to traditional design for print can bring with it a multitude of design sins. Images supplied at 72dpi and crunched down to load fast on a website are going to reproduce very badly in print you can get away with small thumbnails but blowing things up to any appreciable size is going to be pushing your luck. Several online sites are offering free or very cheap quality hi-resolution images which are a good source for suitable imagery.
Forgetting about or not allowing enough bleed.
A very common error is to send to print a document or flattened image that has no bleed at all. Generally speaking, you should allow at least 3mm around every cut off edge. Failing to do so will give the printers no leeway and will either crop off the side of the page or give you a white border. It is always a good idea when supplying image files to save layered PSD files then if things need extending or cropping you can do this on the background layer and hopefully cut down your work
Using obscure fonts and not embedding or outlining them for output.
We’ve all been guilty of this at some point and things are generally fine if you are going to be the only person accessing your artwork or documents. However, if someone else needs to amend the files or use your vector logo on one of their publications. Unless you package up the used fonts, they are not going to be able to open the files correctly and some software programs may replace any unknown fonts with a default. This is a particular problem when you need to dig out stuff that was created several years previously and you no longer have your old fonts installed.
Supplying print ready artwork using spot colours or RGB
There are valid reasons for using spot colours in artwork, logos that need to reference particular Pantone colours for example. In general design work, however, most print is sent through on 4 colour presses CMYK with occasional 5th colour for luminous or metallic colour or spot UV varnish. It is very common for lazy designers to just place RGB images into files and expect the vibrant colours seen on screen to reproduce in print.
Allowing design illiterate clients to lead you around the houses
The customer is always right, goes the adage. However, it is often said with gritted teeth and a sense of patience that recognizes that these morons will at some stage be handing over a fat cheque for your troubles. It is often a good idea when first submitting visuals to throw in a couple of stinkers to hopefully get them to appreciate the design you would like them to accept. There is the very real danger of course of them loving the piece of absolute arsewipe that you knocked up in five minutes to make them think you’ve been earning your money. Still, it’s a living.