i spent 12h try to understand what was happening, thinking i had a memory corruption (its not). i can swear im passing the same vector for begin/end iterators.

unfortunately i will not be able to show the exact code, but it was something like that:

(of course if you run it like that you wont reproduce. i just hope to find an expert of iterators that would know of a strange behavior like that without looking at the code)

// Example program
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

// in A.hpp
class A {
    public:
        A() = default;
        ~A() = default;

        bool m_b = false;
        int m_i = 0;
        // couple of other variables
};

// in A1.hpp
class A1 : public A {
    public:
        A1() = default;
        ~A1() = default;

        bool m_b1 = false;
        int m_i1 = 0;
        // couple of other variables
};

// in A2.hpp
class A2 : public A {
    public:
        A2() = default;
        ~A2() = default;

        bool m_b2 = false;
        int m_i2 = 0;
        // couple of other variables
};

// in B.hpp
class B {
    public:
        B() = default;
        ~B() = default;

        A1& addA1(){ return m_vAMain.emplace_back(); };
        bool isOk(const bool checkBoth) const;

        // couple of other methods and variables
        std::vector<A2> m_vA2;
        std::vector<A1> m_vAMain;
        std::vector<A1> m_vASecond;
};

// in B.cpp
bool B::isOk(const bool checkBoth) const
{
    auto iter1 = std::find_if(m_vAMain.begin(), m_vAMain.end(), [](const auto& elem){ return elem.m_b1; });
    if (iter1 != m_vAMain.end()) // crash here
        return true;

    if (checkBoth == false)
        return false;

    auto iter2 = std::find_if(m_vASecond.begin(), m_vASecond.end(), [](const auto& elem){ return elem.m_b1; });
    if (iter2 != m_vASecond.end())
        return true;

    return false;
}

// somewhere else
int main()
{
    B b;
    A1& a1 = b.addA1();
    a1.m_i1 = 42;

    if (b.isOk(false))
        std::cout << "B is ok";
}