Did anyone notice that PSP titles MotorStorm Artic Edge and Everybody's Tennis disappeared from the PlayStation Store? They went away for a short time after it was discovered that they could be used to run unsigned code on the PlayStation Vita. A homebrew loader called VHBL was released to use this exploit, and it let Vita owners run fan-made games and emulators.
Now both games are back on PSN, as firmware update 1.65 patched their exploit holes. Sony was smooth in quietly pulling these to fix their problem, though it kind of sucks that they pulled these games down without explanation. If you own one of these older games and have been looking to pull it back down lately via PSN, try again now.
Wolool.net says that if you are looking to use VHBL, it should go without saying that these two games will not work anymore. A new version is in the works that is said to be compatible with firmware 1.67, and it seems that this will also use a vulnerability in a PSP game, though they're not saying which this time.