Date: 202104
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Second aircraft: Commercial Fixed Wing
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Second reporter function: Captain
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related; Procedure; Human Factors
Primary problem: Procedure

Narrative: I had just been relieved on Local [positions] combined and this occurred during my two minute overlap and the relieving Controller had the position.  Departure traffic was slow and GCN [Ground Control North] 'Convenience taxiing' Runway XL arrivals at their discretion mostly south on Runway YL to hold short of Runway XC.  The relieving Controller cleared Aircraft Y for takeoff from Runway XC full length.  Aircraft X was taxiing south on Runway YL to hold short of Runway XC (presumably; at the time of this report as I have not heard the audio replay from that position).  The relieving Controller saw Aircraft X continuing towards Runway XC at a rate of speed not consistent with holding short and keyed up and said 'Aircraft X stop stop stop'; which they did.  He then keyed up and said 'Aircraft Y; cancel takeoff clearance stop stop'.  Aircraft Y; said something to the effect of 'Say again' and shortly thereafter rotated.  The ASDE did alert sometime after the relieving Controller told Aircraft X to stop.  Aircraft X appeared from my point of view to be over the hold line and extremely close to Runway XC.  Aircraft Y; rotated near the point of conflict and I believe had Aircraft X continued a collision would have been difficult to avoid.  Aircraft X was then given a crossing clearance.  I then completed my overlap.Discontinue this operation immediately (convenience taxi down Runway YL).  In my opinion this represents the culmination of drift that this facility has acquired during the lower 'COVID traffic' we have experienced in the last year (I am guilty of this too as I did this operation until I saw it not work today).  There are periods of pronounced lulls in traffic (at least departure wise) where it would be poor service not to shortcut arrivals.  However; to taxi on a runway to hold short of a runway I think is now demonstrably unsafe.  I believe it requires the flight crews (and possibly ATC) to digest too much information quickly and perform too flawlessly for little appreciable gain.  We will always have to perform up field crossings due to the layout of the airport; but I believe we should keep them on a taxiway with a good hold short read back as this is a routine occurrence in the NAS and should elicit better performance from flight crews.


Second narrative: After landing on Runway XXL tower cleared us to join Taxiway 1 and contact ground on XYZ.YA [Frequency].   The Ground Controller issued a taxi clearance limit of taxi right hand turn onto Runway 22R -Then hold short of Taxiway 2 - Monitor Tower on XYX.XA [Frequency]. The taxi clearance was written down and verbalized after it was given.   The clearance limit issued only included a hold short of [Taxiway 2]. As we approached Runway XC the flight crew acknowledged an aircraft was in position on Runway XC.   As we were passing Taxiway 3 told us to stop immediately as the aircraft had been issued a takeoff clearance.   We stopped immediately.  The aircraft on Runway XC was issued a cancel takeoff clearance which they did not do and continued the takeoff.   During our monitoring phase of the taxi we were never issued a hold short clearance just the hold short of Taxiway 2 Nor did we hear a takeoff clearance for the aircraft on Runway XC.  Once clear of the conflict - Tower issued a number to contact them.   As we responded the with the read back we heard the aircraft following us tell Tower that they had received the same clearance as we had received.Poor ATC communication and poor clearance limits expectations were the biggest causes of this event.   Flight deck protocol was followed by utilizing sterile flight deck and writing and readying back clearances and following airport charts. An expectation from tower that wasn't issued via the taxi clearance was what the greatest factor contributing To the eventWe debriefed the event and acknowledged even though the taxi limit was followed the importance of vigilance in an high demand taxi environment like ZZZ [Airport] we must be adhered to and even questioned to clarify and avoid the type of scenario to happen as a crew again.


Synopsis: Local Controller who was in the 2 minute relief window observed a runway incursion that resulted in a ground conflict and cited COVID-19 related taxi procedure as a contributing factor. A Captain of one the aircraft also reported the event and stated communication issues with ATC a factor.