My first "deep" dive was my first dive after OW certification. Due to a mix up I ended up without a dive computer but as I was diving in a trio we had 2 identical computers between three (not ideal but not the worst thing). As the others were more experienced, I let them lead and just followed. Did the dive, came back to the surface and while we were swimming back to shore, I asked about the duration and depth and the conversation went something like this:
"33 mins @ 21m".
"Oh - I'm only supposed to go to 18m as I am only OW"
"Oops my bad didn't realise you were only OW"
"Ah OK - enjoyed it anyway"
Point being I didn't realise the depth and apart from being a bit darker, there wasn't really any difference. So long as you watch your gas and computer you will be fine.
Made my first AOW deep dive a bit of a non event though - it was only a couple of metres deeper.