Hello all,
It seems there is a bug in the TAxis::FindBin (Double_t x) method. If one runs the following script,

#include "TAxis.h"
#include "Riostream.h"
#include "TROOT.h"
Int_t GetRightBin (TAxis *ax, Double_t x)
{
//
// Get correct bin number from x value for TAxis ax
//
Int_t retVal = 1+Int_t(ax->GetNbins()*(x-ax->GetXmin())/(ax->GetXmax()-ax->GetXmin()) + 0.5);
return retVal;
}
void TAxisBug (Int_t nBins = 30, Double_t xMin=3.0, Double_t xMax=6.0)
{
//
// Test TAxis bug
//
TAxis *ax=new TAxis (nBins, xMin, xMax);
cout << "Root version : " << gROOT->GetVersion() << endl;
for (Int_t i=1;i<=ax->GetNbins();i++)
{
cout << i << " => " << ax->FindBin (ax->GetBinLowEdge (i))
<< " : " <<
GetRightBin (ax, ax->GetBinLowEdge (i));
if (i != ax->FindBin (ax->GetBinLowEdge (i)))
{
cout << " <= Bug!!! ";
}
cout << endl;
}
cout << endl;
}

one can see that the bin number returned from ax->FindBin (ax->GetBinLowEdge (i)) is not always i as expected.

When looking at the code of TAxis::FindBin (ROOT: hist/hist/src/TAxis.cxx Source File line 314), the formula used to compute the bin number is subject to rounding errors. It should be corrected as propose in the joint script.

This is not really a bug but a limitation due to numerical error when calling FindBin with the bin edge values. Your code will fixes this case but it will return a wrong result when calling FindBin( ax->GetBinLowEdge (i)) - eps) where eps is a value smaller than half of the bin width.
We could maybe provide a better handling of the bin edges cases to avoid this numerical error, but this will slow down the FindBin function which sometimes is used in some critical code. In general , whenever possible, it is always recommended to avoid filling an histogram with bin edge values