cd mastering service engineer's pitfalls1. Not trained hearing skill
This point above is the principal headache of vast number of the newbyes as well as the professional mixing engineers. Really, the lack of skill is the only prevailing serious barrier to produce a high-fidelity mix. The listening skill is one the basic ones every engineer of the online mixing should have. It should be practiced on and on by regular mixing. Doing that your ears will start percieve incomparably greater than ordinary human being can hear. To identify basic equalization, to see the numerous musical effects and not to ball up the instrumental legibility in mix with its rate, - now you have the first acts of being a studio sound master. To be skilled sound engineer, a real pro in the industry one have to hear and identify details of such subtle mix components as audio compression, sound character, soundstage, structure of the overall sound etc. The more regularly you train your listening competency, the better is your nuances hearing becomes.
2. Details neglegence
Concentration and listening are rather essential characteristics to be a sound editor, but here let us speak about thoroughness and a hard work. Engineer's work contains huge number of tiny aspects. Ignoring these details, paying not enough attention to study them and self education as well you are getting at risk to ruin next mix completely. Sounds fancy, yet in reality just couple or even single wrong note, some unpleasant vibrations or the dearth of controls automation at the right place - any of these criterias are able to transmute your song had power to become a fantastic mix into shocking one.
3. Ignorance of toolset
I met people, who alternated several mixing application types (DAWs) one by another in a short time period, who hunted out supernatural plugins while having exceptional ones, people who ignored hotkeys… In a word, I met numerous precedents of resource & time wasting. Infrequently, though a few cases were actually justified. After all, there is no any "magical" digital audio workstations or plug ins, specially designed to convert poorly mixed song into award winning mix. 

mixing engineer's pitfalls4. Unwillingness to Inquire 
Being a audio engineer doesn’t mean that you just have to learn the some limited amount (say invented within last several years) material connected with audio engineering and stop with that. If you desire to succeed in it, try to elaborate and extend your knowledge permanently. Everything is changing. Appearance of brand new music/technological specifications and tends involves non-stop practice. Link and sharing information exchange among your skilled buddies would enforce in your favor also. I am sorry to say, it often happens that a soundman is well informed, his studio is equipped rather good, but results of his work don't impress at all. 
5. Expect on mixing process to fix bugs of poorly recorded audio material

Origin this trap is real not only for mixers, but for arrangers, music producers and recording engineers. The improperly recorded bass or taken with wrong mic voice would not boom as required by with the help of mixing. best online mixing service powerless to correct week arrangement failures. What is it able to make - is to clean up some frequencies, make instrumental lines sound some better but not to transmute mess into fantastic song. The result is actually foreseeable, and you should not blame the mixing engineer for that. 
6. Audio processing 
Sound mixing is the operation by which several separated sounds are treated that way to become a cohesive single stereo track. Quite wrong to confuse it with «processing audio with various effects». There are vast tracks damaged because of sound processing made for itself only. The longer I am working as mixing and mastering engineer the better I understand that amazing sound isn't result of chaoticly put effects yet depends on wise use of treatment made thoroughly. I'd name so many instances, where you can control things just with simple HPF/LPF filtering or rational level/pan adjustments of song components to get a mix, without destroying its reasonable tone. Occasionally to receive an needful sound you must fulfil its processing for times without number. However, the result is worth of spent time for education. If you see something questionable in it, my answer again the same: not quantity first but quality. The most important is to assimilate how to use wisly the sound processing effortlessly. Be purposeful professionals!