As someone who frequently edits over on The Workplace, here is my general opinion on edits:
Suggested Edits Should Fix Everything
Suggested edits create a burden for other users. If you want to help out with a suggested edit, you should make sure that your edit will reduce the amount of effort expended to fix a post. That means dotting your 'i's and crossing your 't's.
Non-Reviewed Edits Should be Significant
Significant is going to be one of those I know it when I see it issues, but here are my personal guidelines.
Titles are what pop up when people search for something on google. This makes them significant because of its visibility. If the title is wrong, fix it. But at least fix it entirely. For instance, in your case:
how to stored serialization Instance(Object) at memcached server?? (JAVA)
Should be at least edited to:
How to store a serialization instance (Object) on a memcached server?
(Apologies if 'on a' is not an appropriate replacement for 'at' in this case, I don't know Java)
This means tags should be removed, punctuation/capitalization should be fixed, and the whole lot of grammatical errors should be fixed.
If you see a post that says:
' Do Stuff Here
Fixing formatting to make it readable has value on its own:
' Do Stuff Here
Fixing Syntax in Answers
Editing code is an entire topic in and of itself, but if an answer (not a question) has a syntax error preventing it from working, adding the missing bracket or semi-colon to fix it will help out anyone who stumbles across it in the future. Just be sure your correction is right (and possibly leave a comment).
Point is, if the answer is broken due to a careless error, fixing the error makes the world a better place even if you have a couple grammar errors scattered throughout.