Note that the array index that is used goes beyond the array's capacity. And thus it modifies the next variable in memory…

weird.cpp

#include <stdio.h>

#define uint unsigned int
#define TTW_NUM_GRANULARITIES 5

class Weird {
public:
    Weird() {
        this->lastUpdate = 0;
        for (int g = 0; g < TTW_NUM_GRANULARITIES; g++)
            this->capacityUsed[g] = 0;
    }
    void increment() { this->lastUpdate++; }
    void test() {
        uint granularity = TTW_NUM_GRANULARITIES;

        printf("before: %d\n", this->lastUpdate);

        // Update this granularity's used capacity..
        this->capacityUsed[granularity] = 0;

        printf("before: %d\n", this->lastUpdate);
    }

protected:
    uint capacityUsed[TTW_NUM_GRANULARITIES];
    uint lastUpdate;
};

int main() {
    Weird w;
    w.increment();
    w.test();
    return 0;
}