One such case is People in interests of EB, 2022COA8 (Colo. App. 2022). The official syllabus of the case states:
In this dependency and neglect proceeding, father appeals the juvenile
court judgment terminating his parent-child legal relationship with
his child. Father contends that the juvenile court abused its
discretion when it denied his request for a continuance after it
learned that he was having technical difficulties participating in the
termination hearing, which was being conducted via Webex
videoconference due to the ongoing COVID-19 pandemic. A division of
the court of appeals concludes that, under the facts and circumstances
of this case, it was an abuse of discretion to deny the requested
continuance. Accordingly, the division reverses the termination
judgment and remands the case to the juvenile court for further
The Colorado Rules of Civil Procedure applicable to the case are substantially identical to the parallel Federal Rules of Civil Procedure of the same number. Harmless error analysis is fairly similar in criminal cases and civil cases, although not quite identical. In theory, the standard in a criminal case to ignore harmless error is stronger than in a civil case. In practice, lots of seemingly significant matters in criminal cases are discounted as harmless error anyway.
The court doesn't explicitly make a harmless error or lack thereof determination, but its analysis makes clear that the error was not harmless.
Issues related to a remote hearing are also discussed in People
in Interest of R.J.B., 2021 COA 4 (Colo. App. 2021), which is cited in this case. The official syllabus to that case tangentially discusses the issue:
In this dependency and neglect proceeding, mother appeals the judgment
terminating her parent-child legal relationship following a remote
termination hearing via Webex. She claims that the court should have
granted her a continuance so an in-person hearing could have been
held, and the remote hearing didn’t afford her due process or equal
protection of the law.
The division concludes that the court didn’t abuse its discretion in
denying the continuance. The court’s need to conduct the termination
hearing via Webex didn’t establish good cause to continue the hearing
when a judge presiding over a hearing held via Webex can address any
technical difficulties with sound, video feed, or broadband issues as
they arise; any delay in making an objection can be redressed by the
court disregarding improperly admitted evidence; the court had
extensively tested the virtual lobby and didn’t allow a sequestered
witness to hear any of the proceeding; Webex, as a real-time
videoconference platform in which all participants may view one
another, allows the court and all counsel to observe a witness’s
demeanor, determine if the witness is relying on documents or other
information, and view admitted exhibits as well as other documents
that may be used for impeachment; and the court ensured that an
official record of the hearing was made in the same manner as during
an in-person hearing.
The division also rejects mother’s assertions that the remote hearing
procedure failed to afford her due process and equal protection of the
law. The division concludes that the juvenile court ensured that
mother was provided substantially similar and fundamentally fair
procedures as would have been available at an in-person termination
hearing. So conducting the termination hearing via Webex afforded
mother due process. The division didn’t consider mother’s equal
protection claim because it is merely a bald assertion without
argument or development.
It is worth mentioning that many cases applying the harmless error standard don't expressly reference Federal Rule of Criminal Procedure 52. Not infrequently, cases don't even mention the buzzwords "harmless error", even though the case makes an analysis that implicitly addresses the issue.
Probably the best place to look for relevant case law would be under annotations to Federal Rule of Criminal Procedure 33 which governs motions for new trials.