Why are some users getting an error when they click on the activation link?

For the most part, we have a lot of successful email verifications. But for some people, they get an error every time they click the verification link in their emails. We have to manually verify these users. I have no idea how only some of these users get the error. I have tested the process multiple times myself and can’t replicate the errors. Does anybody else experience this? Any idea why this might happen?