error plot is very strange

[Lais Arend]

Dear all, 

I've been running well-tempered metadynamics to observe the passage of a ion through a pore and to calculate the FES associated with this passage. As CV,  I've used the distance between the ion and a virtual atom on the other side of the protein, as well as the position of the ion. I've run many replicas, and the FES I've obtained are consistent BUT the problem is with the error plot that I've generated after the block analysis: as the block sizes increase, the error should increase until it reaches a plateau but instead what I obtain is a plot in which the error seems to be increasing in the expected manner until it falls abruptly to zero and there remains. And that's been happening consistenty. I hope some of you guys can come up with a explanation, because I don't know what is happening. 

Thanks in advance :)