Sometimes the possible consequences of connecting old flashes to modern electronic cameras (both digital and film) through hot shoe are discussed. The problem is high voltage (up to at least 300 V) of those flash units on their contacts, exceeding the standard limit of 12 V. I’ve never heard about a destroyed camera from direct user experience, but people are warned not to try such things.
What’s my own experience? An old simple hot shoe flash with voltage about 90 V on its contacts usually worked with my Pentax MZ-M camera, but I sometimes experienced delayed mirror return (for about half a second) after taking a snapshot. I was more cautious with my other body, Pentax Z-1, reported to be sensitive to high voltages. I once attached another flash unit, 40 V, to it. Two snapshots went fine, but on the third one it actually happened what the rumors had warned about: the camera got completely frozen and I had to remove the battery from it to get it alive again. However no other damage has happened.
I no longer risk damaging my cameras by using old flash units. I bought a used Pentax flash unit for €15, with safe voltage. It is only a bit stronger than common built-in flashes, it doesn’t have any TTL etc., but it works, it’s safe, cheap, small, light, inconspicuous and communicates with the Pentax cameras. It’s enough for my occasional flash use.
This is my warning to other photo amateurs: Although no permanent damage has happened to my cameras, I can confirm that using old flashes with voltage higher than 12 V on their contacts may cause at least temporary malfunctions. And it’s not true that Pentax cameras are safe up to 600 V.