As wonderful as special needs trusts are for their disabled beneficiaries, there are a lot of "pitfalls" when it comes to trust management/administration; one wrong step can eliminate the beneficiary's eligibility for government benefits.
One of the biggest pitfalls is a trustee running afoul of the "sole benefit" rule. The rule simply states that the trust must be for the sole benefit of the disabled person. Put another way, any and all distributions from the trust must be limited to those that benefit the disabled person.
If the trustee starts making distributions that benefit other people then the risk is that the government could decide that the trust is no longer a special needs trust, which means that all of the trust assets would be considered "available" to the disabled person and government benefits disappear.
Along these lines, be careful if the disabled person is a minor child with financially viable parents. If a trustee uses trust funds to cover expenses that the child's parents are legally obligated to cover (i.e. basic living expenses), then the government could consider those distributions as benefiting the parents and not the child! The argument in that scenario is that the trust is relieving the parents of a financial burden.
Again, special needs trust rules are confusing and missteps can trigger severe problems for the disabled beneficiary. So, if you're a trustee and you're not 100% sure about the propriety of a proposed distribution, then play it safe (in fact, "play it safe" is a decent mantra for all trustees of a special needs trust!) and consult a professional adviser.