Guide to Grammar and Writing http://webster.commnet.edu/hp/pages/darling/grammar/consistency.htm
A short page offering good examples of shifts in verb tense and "things we have to remember about pronoun reference" along with links to quizzes and other grammar resources.
Misplaced Modifiers http://www.engr.trinity.edu/~Review/1995/gallman24.html
This poem of longing and memory by Geoff Alman includes the lovely line "yet even misplaced modifiers reveal a propensity for intention," which is the reason we point you to it.
A Collection of Ambiguous, Inconsistent, and Incomplete Statements http://www.vuse.vanderbilt.edu/~jgray/funny.html
A treasure trove of inconsistent and incomplete statements from the real world (one favorite: the ad offering "dress shirts for men with 16 necks").