Any line of text that extends to the very edge or beyond the edge of the screen or any two-line subtitle that looks like the lines were split randomly (possibly using word wrap).
While we do have hard limits relating to the max number of characters per line for some languages (see TimedText-MaxCharactersPerLineExceeded), we do not have a hard limit for a large number of languages.
SCC files have a hard limit of 32 characters per row but TTML and STL files can accommodate events of greater length. As a general rule, limiting each line to 42 characters should stop line length being flagged as an issue, however this is not a hard limit and you should rely on your judgement when conforming the asset.
If the issue flagged relates to line breaks that appear random or uneven the issue is likely introduced when you imported your source file into your subtitling software. Most tools have the ability to flag when an event has been word-wrapped during import, ensure this feature is enabled. You can then address each event that is flagged and correct it.