The mistake that only shows up after it is printed
A Google Form has several addresses and they look alike. The one you are almost always looking at is the editor: docs.google.com/forms/d/<id>/edit. It is in the address bar the entire time you are building the form, so it is the one that gets copied, turned into a code, and sent to a printer.
It does not work for respondents. They get a permission wall instead of your questions, and there is no way to tell from the code itself, because a QR code carrying the wrong URL looks exactly like one carrying the right URL. You find out when the posters are already on the wall.
Two other addresses are worse. /viewanalytics is the response summary, so a code built from it shows everyone's answers to anyone who scans it. /formResponse is the endpoint the browser posts to when someone presses Submit; it is not a page and scanning it opens nothing.
The link you want, and why you cannot work it out from the editor link
Open the form, press Send, and use the address on the Link tab. That is the published form, and it looks like docs.google.com/forms/d/e/<different id>/viewform.
Note the /e/ and note that the identifier is not the same one. The published id is not derived from the editor id, so no tool can convert one into the other, and any that claims to is guessing. This one tells you which address you have and where the right one lives rather than pretending.
The short link makes a better code, and this is measurable
Ticking Shorten URL in the Send dialog gives you a forms.gle address of about 25 characters, against roughly 90 for the full one. That is not cosmetic. A QR code holds its payload in a grid, and a longer payload needs a bigger grid: the full address pushes the code to a higher version with more, smaller modules, while the short link fits in a much sparser one.
At the same printed size, fewer modules means each module is physically larger, which is exactly what decides whether a phone can read it from across a room or off a slightly smudged print. For anything going on a poster, a table card, or a lanyard, use the short link.
Prefilled answers survive the trip
If you build a prefilled link from the form's Get prefilled link option, the entry. parameters are part of the address and go into the code with everything else. That is how one printed sheet becomes several codes that each tag their responses differently: one per table, per room, per campus. This keeps those parameters and only drops the usp tracking value Google appends when you copy a link, which does nothing for a scan and only makes the code denser.