The precautionary principle or precautionary approach to risk management states that if an action or policy has a suspected risk of causing harm to the public or to the environment, in the absence of scientific consensus that the action or policy is not harmful, the burden of proof that it is not harmful falls on those taking an action.
The principle is used by policy makers to justify discretionary decisions in situations where there is the possibility of harm from making a certain decision (e.g. taking a particular course of action) when extensive scientific knowledge on the matter is lacking. The principle implies that there is a social responsibility to protect the public from exposure to harm, when scientific investigation has found a plausible risk. These protections can be relaxed only if further scientific findings emerge that provide sound evidence that no harm will result. (from wikipedia)

The Precautionary Principle applied to elections means that even the smallest doubt about e-vote being really less risky than paper voting it's a good reason to use the traditional paper ballots! By the way, what's wrong with ballot papers?

The European Union applies the Principle in the following fields: Consumer protection, Environment, Food safety, Genetically modified organisms, Public health.
Thus EU should apply the Precautionary Principle to elections because

• when we vote we are "consumers" of democracy
• winners of elections are allowed to take decisions that greatly affect both the natural environment and the social environment (which for humans is often more important than the natural one!)

Winners of elections are allowed to take decisions which grately affect everyone, thus I think the Precautionary Principle should be applied to elections as well to ensure that elected politicians really represent the will of the electors' majority.