Burnley were saved from a costly FA Cup mistake when they named a suspended player in their line-up - as opponents Bournemouth pointed out the error.
"It was just a simple admin error," Clarets boss Sean Dyche said.
Since 2018, yellow cards in English domestic professional football have only counted towards suspensions in the competition in which they are received. Pieters was banned for collecting two bookings in this season's FA Cup.
In the past, clubs fielding an ineligible player have been disqualified from the FA Cup or made to replay matches.
Dyche said he was made aware of Pieters' ineligibility by a Bournemouth staff member.