The most common condom mistake? Not using condoms from start (of sexual contact) to finish (after ejaculation)!
The condom should be put on before any genital contact. Some STIs can be transmitted without intercourse, through genital (skin-to-skin) contact. Also, pre-cum can contain semen and STIs (including HIV) so you need to wear a condom the whole time from beginning to end, each and every time.