i know you say no spacers, but do you mean you also didnt have spigot rings to adapt to the centre bore? if not, the wheels would only have been held on by 20 bolts alone, if an aftermarket alloy isnt the correct centre bore the spigot ring to fill the gap is paramount
I do alot of messing about with alloys on cars, i've actually just bought a set yesterday to go on my bora, and this is literally just weeks after changing them before!! I'm on my 8th set in 2 years cos i get bored quickly, but i always ALWAYS check my wheel nuts at the correct time after changing wheels, and at least monthly, on top of random checks when i have a spare few minutes
when you're driving, the bolts are the only thing keeping the car stable on the road in a way!!
while delivering mail down a main road i was distracted by a loud grinding noise, as i looked round i was faced with the front wheel of a transit van coming straight for me which had bounced across the road and thankfully missed everything, until i ran out and kicked it to the floor!
garage had changed a tyre and not tightened his bolts up (his request) and he forgot to do it himself