It can mean, that:

• Your (self set) bins are really not in increasing order, or:
• The number of bins does not correspond to the array you give the histogram to set the bins, for example, you have to set:

double bins[5] = { 1,2,3,4,5 };
double nBins = 4;
TH1D* histo = new TH1D("histo","a histogram",4,bins);