Great question — answering as the UphillTech team:
A pending eTA application is "irrelevant" when applying for admission at a land border. Pending eTA application in itself is not a problem. Many have opt for the land border route if they need to travel but forgot or did not know about the eTA requirement. Some people have names that are unfortunately also associated with other people who are deemed undesirable or of interest by Canada (or another country sharing the many "blacklists"). If they show up on a land border and their other personal information (e.g. age, appearance, sex/gender etc.) clearly demonstrates that they are not the person of interest, they are likely to be granted entry (assuming other admission requirements are met or...
Full answer by xngtng: https://travel.stackexchange.com/a/176198 (CC BY-SA 4.0).
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