This wrong way is No. 1 not because it's the worst wrong way – although it is pretty bad – but because it's hard to believe there are people who do it. After all, everybody knows condoms are disposable ... right?
Apparently, a few people do not. Between 1.4 and 3.3 percent of people reuse condoms during a single session of intercourse, and 1.5 percent of people reuse condoms during different sessions [source: Sanders et al.].
Apart from the yuck factor of reapplying a used condom, condoms just aren't safe to reuse. Sort of like there's not enough eye bleach to rid your brain of some of the most disturbing sex content on the Internet, there's not enough scrubbing to make a condom OK to use again. Any treatment that would kill every sperm and every potential pathogen would also make the condom weaker.
Condom reuse is a bad idea. Use each one – correctly – once and only once, and then wrap it up and throw it away. In a wastebasket or garbage can ... not in the toilet.