Handover failure does not necessarily lead to a call drop. MS is mostly successful in reverting back to the original cell especially in the case of a Better Cell Handover.
Co-BCCH/BSIC case: MS will receive a Handover Command for the cell which is defined as a neighbor instead of the stronger but undefined cell with same BCCH/BSIC. Now MS will try tuning to the new channel of the undefined cell(stronger) but will fail because the info sent in the Handover Command such as MAIO, HSN, TSC etc is not for this cell and BSC hasn’t allocated any channel in this cell. Result is a handover failure with possibility to reversion to old channel.
I am somehow getting a perception that you think handover failure only occurres due to missing definition in OMCR. But this is not the case. Handover failure has many other reasons as explained by Pix in point a and b.