So I had the GPIO diagram upside down (haha). SO I connected the 5V Power pin on the header to the 3.3V Power pin on the GPIO. It was only for a second or less. My RPi still works fine, but I wonder if I should now consider this board suspect?
Did any of the magic blue smoke get out? Also, what was on the other end of the lead that you were connecting to the RPi's GPIO Header? Those supplies on pins 1 and 2 (the end away from the Ethernet/USB connectors are normally to supply power to connected hardware (but the 5V can be used as a power input to the RPi - I have a RPi specific UPS that does that). Depending on what the connected stuff was you might have got away with it provided that the ground connections further towards the middle did not short out the RPi GPIO pins that actually connect to I/O lines on the SoC if they too got swapped over.
Perhaps after this time you are in a better position to judge. If you can come back to us you on this you may be in-line to get a rip tag for this question - as it is I guess this rates an honourable mention! 8-)
1) Take a multimeter and measure 5 volts on the fuse. Both fuse contacts should show the same value
2) Find the MxL7704 chip board and measure the output volts
good video explanation for this: https://www.youtube.com/watch?v=Hunm5S94BaE
Also, to avoid damage to the Raspberry Pi, use an additional separate power supply.
If you accidentally shorted 3.3 volts to the GND and the voltage regulator failed you can buy a new regulator online for $ 3.5 and pay for repair or repair yourself.
Update 23-02-2020: Theoretically as far as i can remember 5 volts goes straight from the power supply to the GPIO raspberry pi. And 3.3 volts goes through the voltage regulator (for example MXL7704-r3) which burns at the closing of 3.3 volts + GND. To prevent the voltage regulator (for example MXL7704-r3) from malfunctioning, do not use 3.3 volts on the Raspberry pi GPIO and instead use a 5 volt to 3.3 converter. For example: