False Super Idiot

"Wow, my gosh! What a change! This has never happened before...huh, wait, it's the legend! Goku must have transformed into a Super Saiyan!"

- King Kai (Dragon Ball Z: Lord Slug)

False Super Saiyan (疑似超サイヤ人) is a Saiyan transformation that Goku takes during his battle against Lord Slug in movie 4, Dragon Ball Z: Lord Slug. In the anime Daizenshuu, this form was originally labeled as the true Super Saiyan form, but later re-labeled as Giji or "False" Super Saiyan after the actual Super Saiyan form was introduced in the manga series.

Appearance
In this form, Goku's irises and pupils are no longer visible, mostly in turn to the power and bleak rage. His body bulks up a little, his hair straightens like that of a Super Saiyan, his skin color also appears to have a yellow hue to it, but there is no considerable change in feature other than a red tint to his hair (due to the reflection of the yellow aura on his black hair, his hair is not actually red). He gains a yellowish surge of aura, somewhat but not quite resembling that of a normal Super Saiyan's. In turn, the form somewhat resembles a glorified Kaio-ken form and even makes the same sound as it. However, there are notable differences.

When he transformed into the true Super Saiyan during the fight against Frieza, Goku witnessed the murder of his best friend, Krillin (as well as the fact that Krillin is not likely to be revived with the Dragon Balls, as Shenron had already granted that wish and could not resurrect the same person twice), something that drove him over the edge. This ultimate pain was not fully present during the fight against Slug, that and Goku wasn't at the same power level as when he fought Frieza, which is most likely why Goku did not transform fully. The form of False Super Saiyan was shown to be very erratic, lasting for only as long as the aura is ignited, and using up much more energy to sustain than the full Super Saiyan.

Usage
During the battle against Lord Slug, Goku's strength began to quickly debilitate. Having no more options, Goku's anger surged, causing him to gain a massive power increase, enough to quickly overwhelm Slug. However, after a quick sequence of thrashing Slug around, Goku's False Super Saiyan power quickly faded.

One way to put it is that the False Super Saiyan transformation occurs when a Saiyan has the required physical power but lacks the required mental strife to transform into a Super Saiyan. It is apparent, however, that the form gives a large power increase, enough for Goku, who was being easily beaten previously by Slug, to now quickly overwhelm him.

Trivia

 * King Kai said this transformation is the Super Saiyan transformation.
 * A popular misconception among fans is that Future Trunks once achieved the False Super Saiyan state due to the similar appearance of the state he took in both the Dragon Ball Z: The History of Trunks TV Special, and just before fighting Frieza. However, Daizenshuu 7 only list Goku as a user of this transformation, even though both Gohan and Future Trunks display similar characteristics of the form in the anime.
 * A similar misconception involves Goku's "semi-Super Saiyan" state during the filler anime episode, "Warriors of the Dead". In this case, however, it was a last-minute decision to keep Goku's Super Saiyan powers hidden from Pikkon rather than a simple coincidence, thus his hair was digitally altered from gold to black and his eyes from green to black. (In the original Japanese promo for the episode, Goku's hair turns gold and eyes turn green.) The aura was oddly untouched.
 * Broly also displays a similar form in Paragus' first flashback sequence during the movie Dragon Ball Z: Broly - The Legendary Super Saiyan; in which a teenage Broly is running amok, destroying planets unrestrained. The form's hair stands up on end like Broly's Super Saiyan form and has the same reddish tint to the hair as Goku's False Super Saiyan form, however his eyes retain their pupils. Also, Broly exhibited a similar transformation when escaping the destruction of Planet Vegeta due to his eyes going blank.