The Wales full-back and the Ospreys have both been summoned to appear before a disciplinary panel following last Saturday's 16th-man controversy against Leicester.
Byrne returned to the field from the blood-bin during the second half of the Ospreys' 17-12 victory without the Welsh region withdrawing his temporary replacement Sonny Parker.
The Ospreys had 16 men on the field for almost a minute, during which time Byrne was involved in a tackle on Leicester scrum-half Ben Youngs. The Tigers believe Byrne stopped a potential try and submitted an official complaint to Heineken Cup organisers immediately after the match.
Disciplinary officer Roger O'Connor launched an investigation and yesterday ordered Byrne and the Ospreys to a hearing in Dublin tomorrow.
If found guilty of misconduct, Byrne faces the possibility of a suspension which could rule him out of Wales' clash with England at Twickenham on 6 February.
Today's developments are potentially bad news for Wales coach Warren Gatland, who has earmarked the Lions star for a return at full-back after missing the autumn series through injury.
Meanwhile, the Ospreys will be left worrying about a possible fine. England were hit in the pocket in 2003 after being found guilty of a similar offence during a Rugby World Cup pool match against Samoa. Anything more severe, such as a potential points deduction, could wreck the Ospreys' European title ambitions.
Scott Johnson's side booked their quarter-final place as one of two best pool runners-up, finishing behind Clermont Auvergne but two points above Leicester.
Chaos ensued at the Liberty Stadium when Byrne rejoined the action.
Referee Alan Lewis was seemingly unaware he had returned and berated an Ospreys touchline official. Lewis should have awarded Leicester a penalty once Byrne's reappearance became evident, but it is understood the official may have been told by an unnamed Ospreys player that Leicester also had 16 on the field.
Leicester moved quickly after the final whistle, with club chief executive Peter Wheeler and chairman Peter Tom both confirming a complaint had been made within an hour of the game finishing.
Tom was also due to write directly to ERC chairman Jean-Pierre Lux, while the Tigers have hired a team of specialist sports lawyers to head up their protest.
The 16th-player episode comes with dust only just settling on last season's big Heineken Cup controversy. That was the "Bloodgate" episode during a quarter-final between Harlequins and Leinster, when Quins wing Tom Williams was ordered to fake a blood injury in an attempt to get goalkicker Nick Evans back on the field during a game Quins lost 6-5.
After a lengthy ERC investigation, Williams, former Quins rugby director Dean Richards and ex-club physiotherapist Steph Brennan were all banned for varying periods.
Richards and Brennan have both left Quins, while Williams is now playing for them again after completing his suspension last November.