I know this is going to make me very unpopular saying this, but it was also totally possible to upload all your documents within minutes of it opening without any errors or missing documents. I know I (I think anyway...) and some other posters like Alberta2018 did this. I think Alberta2018 said he went out and bought himself a printer and scanner just for this. I spent about 4 solid days in the run-up to the portal opening filling in all the usual forms, triple checking and then triple checking again that all the dates were correct and I had all the information. I printed, signed and scanned everything just in case... I paid my fees the night before. When the checklist became available all I had to do was complete the additional pathway specific form and I had to go through a couple of my forms and write 'not applicable' or whatever the guidelines were in some blanks. I've been preparing for Canadian Immigration since I first came to Canada just in case... I requested my PCC for my home country when I left just in case, I kept my language test up to date just in case... etc. The only real fk-up IRCC did was to not say if we needed a medical exam in advance or not until the day before (I had mine done just in case...) but they let us upload our application with just an appointment request and letter of explanation.
I know this isn't being helpful or anything, but I'm just saying - we can't keep using the excuse that we were rushed and so we 'forgot' we'd been denied entry to the USA, or whatever. I think the people that made the mistakes are the ones looking for assistance on this forum so it gives the impression everybody made mistakes, but in reality there are probably thousands that didn't.