Index exceeds matrix dimensions

data_video = zeros(conf{layer,1}.filt_h*conf{layer,1}.filt_w*conf{layer,1}.in_frames ...
        *conf{layer,1}.in_fmaps, num_patch); %zeros(15*15*3*1,1000)

    for idx = 1:num_patch %num_patch is 1000
        data_idx = ceil(rand(1,1)*num_data); %num_data is 900
        data_fone = train_video{data_idx,layer};
        n_hframe = size(data_fone,1); %let's say 26
        n_wframe = size(data_fone,2); %let's say 70
        hframe = ceil(rand(1,1)*(n_hframe-conf{layer,1}.filt_h+1)); 
        wframe = ceil(rand(1,1)*(n_wframe-conf{layer,1}.filt_w+1));

        %patch from frame1
        data_fone = reshape(data_fone(hframe:hframe+conf{layer,1}.filt_h-1, ... 
            wframe:wframe+conf{layer,1}.filt_w-1, :), ...
            config{layer,1}.filt_h* conf{layer,1}.filt_w, 1);
        %patch from frame2
        data_ftwo = reshape(train_video{data_idx+1,layer}(hframe:hframe+conf{layer,1}.filt_ht-1, ... 
            wframe:wframe+conf{layer,1}.filt_w-1, :), ...
            conf{layer,1}.filt_h* conf{layer,1}.filt_w, 1);
        %patch from frame3
        data_fthree = reshape(train_video{data_idx+2,layer}(hframe:hframe+conf{layer,1}.filt_h-1, ... 
            wframe:wframe+conf{layer,1}.filt_w-1, :), ...
            conf{layer,1}.filt_h* conf{layer,1}.filt_w,1);

        %concatenate the patches from three frames
        data = [data_fone; data_ftwo; data_fthree]; %675x1

        data_video(:,idx) = reshape(data, ...
            conf{layer,1}.filt_h * conf{layer,1}.filt_w * conf{layer,1}.in_frames ...
            * conf{layer,1}.in_fmaps, 1);          
    end

when I run this, it says 'Index exceeds matrix dimensions'. And it doesn't produce error at the same index number every time I run it. Why is that? Where is the error here?