Example code to reproduce the error in ofxFastFboReader (https://github.com/satoruhiga/ofxFastFboReader) I discovered. Create an app, resize the window and press "s" to save a different file in your data directory. Sometimes this will work and other times it won't.

testApp.cpp

#include "testApp.h"

//--------------------------------------------------------------
void testApp::setup(){
    ofSetVerticalSync(true);
    ofEnableAlphaBlending();
    ofSetFrameRate(30);
    
    counter = 0;
    fboReader.setAsync(false);
    
    rect = ofRectangle(0,0,15,15);
    speed = ofVec2f(5,5);
    pos = ofVec2f(10,10);
}

//--------------------------------------------------------------
void testApp::update(){
    if(bSavingImage) {
        saveImage();
        bSavingImage = false;
    }
    
    pos += speed;

    if(rect.y+rect.height >= ofGetWindowHeight() && speed.y > 0) {
        speed.y *= -1;
    }
    else if(rect.y < 0 && speed.y < 0) {
        speed.y *= -1;
    }

    if(rect.x+rect.width >= ofGetWindowWidth() && speed.x > 0) {
        speed.x *= -1;
    }
    else if(rect.x < 0 && speed.x < 0) {
        speed.x *= -1;
    }
    
    rect.position += speed;
}

void testApp::prepareToSaveImage() {
    bSavingImage = true;
}

void testApp::saveImage() {
    ofFbo fbo;
    fbo.allocate(ofGetWindowWidth(), ofGetWindowHeight(),GL_RGB);
    fbo.begin(); {
        draw();
    } fbo.end();
    
    ofPixels px;
    fboReader.readToPixels(fbo, px);
    
    ofImage img;
    img.setFromPixels(px);
    
    img.saveImage(ofToString(counter) + ".png");
    counter++;
    
    cout << "saved." << endl;
}

//--------------------------------------------------------------
void testApp::draw(){
    ofBackground(0);
    ofSetColor(0,255,0);
    
    ofRect(rect);
    
    if(!bSavingImage) {
        ofSetColor(ofColor::white);
        ofDrawBitmapString("Press s to save.",20,20);
        ofDrawBitmapString(ofToString(ofGetWindowWidth()) + " x " + ofToString(ofGetWindowHeight()), 20,40);
    }
}

//--------------------------------------------------------------
void testApp::keyPressed(int key){

}

//--------------------------------------------------------------
void testApp::keyReleased(int key){
    if(key == 's') {
        prepareToSaveImage();
    }
}

//--------------------------------------------------------------
void testApp::mouseMoved(int x, int y ){

}

//--------------------------------------------------------------
void testApp::mouseDragged(int x, int y, int button){

}

//--------------------------------------------------------------
void testApp::mousePressed(int x, int y, int button){

}

//--------------------------------------------------------------
void testApp::mouseReleased(int x, int y, int button){

}

//--------------------------------------------------------------
void testApp::windowResized(int w, int h){

}

//--------------------------------------------------------------
void testApp::gotMessage(ofMessage msg){

}

//--------------------------------------------------------------
void testApp::dragEvent(ofDragInfo dragInfo){ 

}

testApp.h

#pragma once

#include "ofMain.h"
#include "ofxFastFboReader.h"

class testApp : public ofBaseApp{

  public:
		void setup();
		void update();
		void draw();

		void keyPressed  (int key);
		void keyReleased(int key);
		void mouseMoved(int x, int y );
		void mouseDragged(int x, int y, int button);
		void mousePressed(int x, int y, int button);
		void mouseReleased(int x, int y, int button);
		void windowResized(int w, int h);
		void dragEvent(ofDragInfo dragInfo);
		void gotMessage(ofMessage msg);

        bool bSavingImage;
        void saveImage();
        void prepareToSaveImage();
        ofxFastFboReader fboReader;

        unsigned int counter;

        ofVec2f pos;
        ofVec2f speed;

        ofRectangle rect;
};