Lowest difficulty too difficult for the given drain time(s)

osu!, osu!taiko, and osu!catch: below 2:30 at most Normal, below 3:15 at most Hard, below 4:00 at most Insane.
osu!mania: below 2:00 at most Normal, below 2:45 at most Hard, below 3:30 at most Insane, for each key mode separately.

In osu!, osu!taiko and osu!catch, break times may be combined with drain time, limited to at most 30 seconds for the highest difficulty. This does not apply to difficulties with less than 30 seconds of drain time.

In osu!catch and osu!mania, a proper spread of at least 4, 3, or 2 difficulties (for each respective threshold) can be provided instead. Since whether a spread is proper cannot be determined automatically, this is left as a warning to verify manually.

Reasoning

Newer players usually struggle with especially long songs, so encouraging them to try shorter songs first at lower difficulty levels allows them to learn the basics before trying to train their stamina or similar. This is done by requiring that shorter songs have lower difficulties, while longer songs can have less of them. This also reduces the workload on mappers and as such introduces a larger variety of songs into the game that otherwise wouldn't be so common due to their length.

Outcomes

Problem
With a lowest difficulty lowest diff, the drain/drain + break time of beatmap must be at least lowest drain, currently current drain.
Cause: The lowest difficulty of a beatmapset (or key mode, for osu!mania) is too high of a difficulty level considering the drain time of the difficulties, and there are not enough difficulties to rely on a proper spread instead.

Spread
With a lowest difficulty lowest diff, the drain/drain + break time of beatmap must be at least lowest drain, currently current drain. Ensure the difficulty count difficulties form a proper spread.
Cause: Same as the problem, except there are enough difficulties that a proper spread (osu!catch and osu!mania only) could satisfy the requirement instead.

Chord too big for its difficulty Multiple reverses on too short sliders

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