condom effectiveness

just a general question.  I know that it is said that condoms are 99% effective in protecting one from a std or hiv.  What causes the 1% of failures?  

[Teak]

Latex condoms, when used consistently and correctly, are highly effective in
preventing transmission of HIV, the virus that causes AIDS. In addition, correct
and consistent use of latex condoms can reduce the risk of other sexually
transmitted diseases (STDs), including discharge and genital ulcer diseases.
While the effect of condoms in preventing human papilloma virus (HPV) infection
is unknown, condom use has been associated with a lower rate of cervical
cancer, an HPV-associated disease. Laboratory studies have demonstrated that latex condoms provide an essentially impermeable barrier to particles the size of STD pathogens.

THE USER!!!

[yarrak]

If you don't put on the condom properly or use oil based lubricants, then the condom protection efficiency can't be 1oo%, but in other cases it provides full protection against HIV transmission. There is no single record of any person infected during a protected intercourse with HIV+ people (note that there are too many HIV+/HIV- couples under regular survaillance and not a single case of seroconversion among HIV- partners).