Saying sorry once you made a mistake in your blog simply wouldn't justify what has been done. Your mistake is going to be expensive as well as detrimental. You cannot simply sum up the number of mistakes you can make when it comes to blogging. If you do a bit of research online and read some articles you will find out there are more than fifty common mistakes most of the bloggers unknowingly make.
Your blog must communicate why the reader should take any interest, very fast. Given the fact that yours is not the only blog around and the readers' attention is short-spanned your design must talk for itself of what it is all about.

Do not use fancy and ridiculous fonts in your site. No matter how attracted you are to them your posts should be readable if you intend to build an audience. Furthermore do not use miniscule fonts either. People will not spend time squinting their eyes to simply read your posts. Make sure you make your readers comfortable because your entire blogging future depends on how you treat them.

Also do not set your settings to resize the reader's browser. Not only you, everyone likes to be in control of their own browser.
Do not force people to register with you unless it is really necessary, so that will make them come back for more, besides subscribing your readers automatically for something without their consent will diminish your credibility with your readers.

Load time is one of the main facts that you should be concerned about. Therefore overusing Flash can make your site take more time to load in addition to the normal time it takes to load. People easily get ticked off if they have to wait for too long.

Your preference of music may not be everybody's favorite. Hence do not try to integrate music in to your site.

Make sure your posts have appropriate titles and tags which will be an essential point when handling SEO. Even if you are using a blog automation programme (eg: [http://www.blogwarriorelite.com]) please double check whether they provide fitting titles and tags for your posts.
If can, avoid inserting drop down menus. Be straightforward with your readers and show them what is on offer at one glance.

These are only a very few common mistakes most of the bloggers neglect unknowingly but not all. so watch your step!