What would happen if everyone on Earth flushed their toilets at the same time?
Probably nothing, besides a few thousand sewer busts, lots of stincky gloop of everything that, well you get the picture. Don't you? If not, sorry. I hope that this answers your question. If not, sorry. Do not try this unless you live on high ground. The people on the dark side would lose some sleep. The earth would suddenly be deafened by a great noise: half the population of the earth would yell at the other half "be sure to put the seat down!" I don't think anything would happen, except for the momentary drop in water pressure as the toilets refilled. BTW, not every culture uses toilets that flush.