A "sting in the tail" could be looming for flexible drawdown clients who could face a HMRC fine of £300 or more.
That was the warning from platform provider AJ Bell, which said advisers should be wary about changes coming in on the day the reforms take effect - April 6.
The warning surrounds the Money Purchase Annual Allowance.
Failure to meet certain reporting requirements relating to this within 91 days could result in a fine of £300 plus £60 for each day by which the eventual report was late.
Savers who flexibly access their benefits after 6 April will be subject to a reduced £10,000 allowance.
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Those savers who then go on to make contributions are required to tell the providers of those schemes that they are subject to the £10,000 allowance.
To ensure that most savers are aware of the reporting requirements, when a saver first flexibly accesses their pension, their provider is required to confirm details of the reporting requirements that the saver must meet.
AJ Bell technical resources manager Gareth James pointed to the potential problem, however, that there is a gap in the requirements.
This means, he said, that there is no requirement for providers to tell customers who are already in flexible drawdown of the reporting requirements.
Those already in flexible drawdown will be able to make contributions of up to £10,000 from 6 April 2015 and must meet the reporting requirements if they start to make contributions.
Mr James said this means the saver could make contributions without being aware that they need to confirm to that scheme that they are subject to the reduced £10,000 allowance.
He said: "There are potentially tens of thousands of savers in flexible drawdown who have not been able to make contributions for a number of years who will suddenly be able to pay up to £10,000 to money purchase schemes. This will be attractive to many, but will come with a sting in the tail in the form of a HMRC fine if they fail to meet the reporting requirements."
He said: "We will be communicating with advisers who have clients in this situation to make sure they are aware of the requirements, I would hope that other providers will do the same. It would be optimistic for HMRC to assume that all providers will, or that savers will somehow know about the reporting requirements without direction of some sort."