"Reverse connectivity," often abbreviated "revcon" or "RevCon," is a situation where a hidden turn is allowed going the wrong direction into a one-way street (against traffic), or leading the wrong direction out of a one-way street.
A connection (turn allowed) from one segment to another via a junction which goes against the directionality of at least one of the segments is called a Reverse Connection, and these segments have "reverse connectivity." This can cause routing irregularities.
Though it is hard to see this in the editor, two segments of a one-way road connected together could have an allowed "turn" in the opposite direction of the flow of traffic, or directionality, of the segments. This can be seen in the editor by turning both segments to 2-way and then seeing the location of the allowed turns. Several user scripts can highlight revcons, including the Color Highlighting userscript, WME Toolbox, and WME Validator. For R5/Country Managers, WME Toolbox can also perform one-click fixes of revcons.
Revcons can also affect routing instructions, by causing the routing engine to see a possible route where none is allowed. This may cause it to issue an unnecessary but valid instruction, a questionable instruction, or, in unusual circumstances, increases the likelihood of creating an illegal routing plan.