Yes, really intense exercise can pose a risk.
But heart rate is not the best measure of exercise intensity for everyone, for reasons that SERGEANTMAJOR has pointed out.
A high heart rate that is the result of working out really hard on the Rate of Perceived Exertion scale and/or failing the 'talk test' may be an issue.
A high heart rate but a low RPE may suggest that heart rate is not the best measure for that particular person. www.sparkpeople.com/resource/fitness_artic
details the Rate of Perceived Exertion and other ways to assess workout intensity.