A fast PCD8544 library (Nokia 5110)

Using Arduino Displays
24

framebuffer variant has a hidden bug

size_t PCD8544_SPI_FB::write(uint8_t data)
{
	// Non-ASCII characters are not supported.
	if (data < 0x20 || data > 0x7F) return 0;

	memcpy_P(this->m_Buffer + this->m_Position, ASCII[data - 0x20], 5);
	this->m_Buffer[this->m_Position+5] = 0x00;   
	this->m_Position += 6;
	if (this->m_Position >= BUF_LEN) this->m_Position -= BUF_LEN;
	//this->m_Position %= BUF_LEN;
	return 1;
}

before the memcpy you need to check that m_Buffer + this->m_Position ... m_Buffer + this->m_Position +5 are valid positions ....
otherwise you copy beyond the buffer ...