BattleMech Manual (p. 87)
’Mechs designed with this quirk do not follow the existing construction rules for some reason, achieving effects that are not normally allowed in game-play, and which can fail spectacularly at any moment. (Note: The total weight and critical space of all components must still fit within the design’s tonnage and critical space.)
If employed, the opposing player may roll 2D6 to determine if the ’Mech with this quirk suffers a catastrophic failure once every 6 turns of combat. On a result of 6 or higher, such catastrophic failure occurs in 1D6 of the ’Mech’s hit locations (using the Front/Back Hit Locations Table). A Critical Hit check is then made for each failure location determined, applying a +4 modifier to the roll result. On any modified roll result of 13+, the location suffers complete collapse if it is an arm, leg, head, wing, main gun, or turret. This will destroy all items within (and any explosive components checks should be made as appropriate for location destruction). If the affected location is none of the above, it suffers 4 critical hits, determined as normal.
Illegal ’Mechs suffer an additional +4 Target Number modifier to repair or replace any damaged or destroyed components, and double all repair and maintenance times for work performed on them. If an Illegal ’Mech repair fails, the item gains the Non-Functional quirk as well.
Finally, because they are technically failures at the time of their production, Illegal ’Mechs should also receive the Obsolete quirk, with the year of their obsolescence determined to be the same as their year of introduction.