Witch Responds: To cut to the chase: NO, it is absolutely not true.
Retrogrades are a normal and, of course, perfectly natural occurrence. At almost all moments of the year, one or more planets are in retrograde. Mercury has simply been in the pop culture spotlight because of how strongly it impacts technology, communication and travel. Of course, these things we always notice in our modern daily lives.
Backfiring is a myth. What is real is poor planning and poor judgement. For example, cursing your ex-husband will likely lead to him being unable to keep up with child support payments. This may put you in an unstable financial situation.
The spell did not backfire. The curse simply wasn't thought through and was a bad decision.
Take 100% responsibility for the work that you do. Thoroughly think through the (potential) outcome and how it will directly and indirectly affect you. Don't curse the hand that feeds you.
Love & Magick,