I have had two separate issues when attempting to merge leads. Most leads are merging with no issues, but two pairs of leads keep failing to merge. I have received two different error numbers (27954, and 73126).
Does anyone have experience with this or know of why these leads will not merge?
Are these leads sync'ed with your CRM? It is possible that the CRM merge fails, which in turn causes a merge failure in Marketo.
If you're using MS Dynamics, then you can't do the merge in Marketo - you'll have to do it in Dynamics.
If you're using SFDC, most often the reason is a permissions issue with the sync user or some other issue communicating with one of the merging records (for instance, if one was deleted from SFDC but the sync never saw the deletion to remove the dependency). You should file a Support ticket if you're stuck.