A couple of years ago I mentioned an article reviewing antibody selectivity

In 2011, an evaluation of 246 antibodies used in epigenetic studies found that one-quarter failed tests for specificity, meaning that they often bound to more than one target. Four antibodies were perfectly specific — but to the wrong target.

The issue of antibody selectivity has again been flagged as a concern in oestrogen receptor beta research DOI. This is a major target for breast cancer research and there are multiple ongoing clinical trials https://clinicaltrials.gov/ct2/results?term=ERβ.

We here perform a rigorous validation of 13 anti-ERβ antibodies, using well-characterized controls and a panel of validation methods. We conclude that only one antibody, the rarely used monoclonal PPZ0506, specifically targets ERβ in immunohistochemistry.

Applying this antibody for protein expression profiling in 44 normal and 21 malignant human tissues, we detect ERβ protein in testis, ovary, lymphoid cells, granulosa cell tumours, and a subset of malignant melanoma and thyroid cancers. We do not find evidence of expression in normal or cancerous human breast.

Perhaps more worryingly the authors comment.

While our study focuses on ERβ, we do not think that antibodies towards ERβ are significantly poorer than those targeting other proteins, and it is not unlikely that this problem generates similar obstacles in many other fields.

As I wrote on the Target Validation page

Remember this is an absolutely critical step, almost everything else can be fixed.