False Positive

From Wikipedia, False Positive is defined like this:

The error of rejecting a null hypothesis when it is actually true. In other words, this is the error of accepting an alternative hypothesis (the real hypothesis of interest) when the results can be attributed to chance

Also some example of False Positive:

A false positive normally means that a test claims something to be positive, when that is not the case. For example, a test saying a woman is pregnant when she is actually not pregnant is an example of a false positive.

Today, GMail had a false positive on one of the email sent to me and marked it as SPAM, so it would put it on Spam directory (as usual). The terrible thing is that i clicked the Delete all spam messages now links, which will delete the email messages marked as Spams forever (not even saving it in Trash). The problem is that one of the email that was supposed to be my student's task was marked as SPAM, so it get wiped out together with the rest of the spams

I probably have to ask them to resend it again. Next time, i will be more careful when deleting emails, mostly emails that was marked as SPAM. Sometimes, it's not a spam and filters can be wrong.