Here we will see some common problems while modifying blogger templates.If you using blogger platform this will be useful for you.Before edit your blogger template first take backup otherwise you will face below problems.And sometimes it will also affect SEO because of 
unwanted code.
Before you are editing the template ensure that please checkbox is ticked otherwise sometimes you can’t find the code which you want to modify.
Problem:
Sometimes you are trying to add some css or html code(widgets) from other websites that time in case the code doesn’t contain end tag below problem will come.
Your template could not be parsed as it is not well-formed. Please make sure all XML elements are closed properly.XML error message: The element type "b:include" must be terminated by the matching end-tag
Solution:
If you facing above problem ensure that you have end tag like below.
<b:includedata=’blog’name=’all-head-content’/>
Sometimes you will face Browser Incompatibilities problems.for example your blog design will work for IE and Chrome but it won’t work in Mozilla or vice versa.I was faced this kind of problem.
Solution
Change the code it should be suitable for all the browsers.
Problem:
Sometimes your template won’t support some widgets or features like Gravatar comment section.
Solution:
Refer some good websites for blogging tips.There you can find solution for that problem otherwise try equivalent widget while is suitable for your blog design.
Problem:
Meta tags problems.
Solution:
Meta tags are more important for SEO.So always place your meta tags within head section and below title tag.
Problem:
Sometimes you can see after editing the templates sidebars of your blog came to below post.You can see this problem in some browsers.
Solution
For this problem ensure that you closed all the tags in previous changes or your might paste the code in wrong location.
Also validate your blog with W3C Validation Service and Check this link it will be useful for you Common HTML Problems.
Still if any problem persist check this link it might be blogger known issues.