I love using interns for this. Typically they 1. Have some free time to test things out. 2. Smart enough they won't try to eat the mouse. 3. Dumb enough they won't be able to figure out those 20 obscure steps to make it work like the senior dev would.

It won't find all bugs, finding ui/ux issues or common bugs they are great at. One example that I can think of is our badges had 2 barcodes on them, one assigned by new company who bought us and one assigned by original company(for legacy and regulatory reasons). If you never worked for old company the second bar code was all zeros. Most employees knew this and new to always scan the new one to log into software. An intern didn't. They ran a bunch of tests but reviewing the data the "operator" was listed as all zeros. This could have been a legal issue if released into production. Thankfully it was caught because intern didn't know which bar code to use, a check was implemented in the code to ensure the user was not all zeroes(as well as other tests to make sure it was new company and not old company but I can't go into detail on those).