A Warning message often occurred before a boss fight in the Mega Man series, starting with Mega Man X4. It generally has an alarm sounding with the word "WARNING" appearing on the screen, sometimes a voice saying the word repeatedly. This was added in largely to mask the loading times of the disc-based games.
Mega Man series
Although the warning did not occur in any of the Mega Man classic games, it was included in the remake Mega Man Powered Up, when the player was about to fight one of the bosses. Similar to the Mega Man X series (exempting X7 and Command Mission), the warning generally appears before the pre-battle cutscene. Mostly because of the lighter and softer tone of the game and series, especially in comparison to the succeeding series, the warning is just composed of bells ringing in a siren-like manner and red flashing intermittedly before the "warning" tone appears in a slightly childish font with red and a barely noticeable yellow border and the announcer yelling "Warning" in a high-pitched voice before disappearing in a flash of light.
Mega Man X series
Although the warning did not occur in the original Mega Man X, it was included in the remake Mega Man: Maverick Hunter X. In that case, the warning logo appears in red at the center, with two pairs of yellow bars forming at the top and bottom, the outer bars forming together to form a scrolling message in red text saying "Target Approaching", and the inner bars, much thinner, just being above/below the Warning label itself and having three alarm blarings before the warning message ends.
Its true debut was in Mega Man X4, where it had a periodical buzz warning as the letters spell out "Warning", the screen flashes, and then the letters disappear again. It will continue to be used in this manner up to Mega Man X6. (In a prototype build of X6, a voice saying "Warning" was also included; this was removed in the final version of the game.)
In Mega Man X7, the Warning is depicted as a purple border with a flashing red Warning sign in the middle, as well as the announcer announcing the word twice. It is also the only standard Mega Man X game to place the warning immediately before the battle instead of simply before the pre-battle scene, something that would be more prevalent in Mega Man Zero onwards.
In Mega Man X8, the Warning is depicted as a flash of red as well as the warning label popping up, with the announcer announcing the word twice.
In Mega Man X: Command Mission, the warning letters will fly by before forming the words as the alarm starts blaring. Because of the nature of the game being an RPG, it also takes place immediately before the battle instead of the pre-battle cutscene.
In this series, the warning segment plays before conversation with the boss battles, except in X7 and Command Mission (the latter due to the RPG nature of the game).
Warning also appears before bosses in Rockman X DiVE, and a "Caution" message appears before sub-bosses.
Mega Man Zero series
Starting with the Mega Man Zero series, the warning will play immediately before the battle starts, instead of prior to the cutscene where the boss arrives. In each installment, the warning sign appears with the letters forming before lines appear above and below the warning sign, and then a three-boop sound effect is heard before the warning disappears.
Mega Man ZX series
Like in the Mega Man Zero series before it, the warning will play immediately before the battle starts.
In Mega Man ZX, the warning label appears, with the screen briefly flashing red in a static-like manner as a long drawn-out alarm is heard.
In Mega Man ZX Advent, the warning label appears, with the screen flashing red twice in sequence to an alarm blaring.
A warning message appeared before a boss battle in Street Fighter × All Capcom.
A warning message appears in Super Smash Bros for the 3DS and Wii U as well as Super Smash Bros. Ultimate when a new fighter appears upon meeting certain conditions for unlocking the fighter.