Long expressions can extend beyond their containers:
\(\displaystyle\sum_{n=1}^{10} {1\over n} = {1\over 1} + {1\over 2} + {1\over 3}
+ {1\over 4} + {1\over 5} + {1\over 6} + {1\over 7} + {1\over 8}
+ {1\over 9} + {1\over 10}\)
You could turn on linebreaking to solve the problem, but may prefer not to.
Instead, you can rescale the math after it has been typeset.
