I've been researching my family tree for a few years now.
The dead ends I have reached have almost all been down to the woman taking her husband's name on marriage.
Back as far as 1837 when civil registration came in, it is usually possible to look up what a baby's mother's birth name was, although sometimes it isn't listed or is spelled incorrectly. But before that, unless you can find parish marriage records showing what the woman's birth name was, you hit a dead end. All you know is that Jane Smith's parents were called John and Mary Smith, which means you can generally go no further back in the maternal line. Looking for a Mary Ann in East London with only an approximate age is like looking for a needle in a haystack.
It's much easier searching French records and even some Scottish parish records where the mother's birth name is also given.
A lot of what we think we know about our ancestry needs to be taken with a pinch of salt as well. I have an ancestor who was married three times, including once probably bigamously, who lied about her age the first time she got married and was listed under a different surname or even Christian name on each census and lied about being married/how long she had been married on at least two censuses. Tracing her was an absolute nightmare.
Then there's the fact that a child'a biological parent or parents might not have been who they supposedly were. Not just children who might have been fathered by a man other than the mother's husband, either due to infidelity or rape or due to the parents meeting after the child was born and pretending they were the father's biological child, but also illegitimate children being raised by grandparents. There are quite a few women in my family tree who supposedly had children well into their 40s when their eldest unmarried daughters were in their teens or early 20s. So most genealogy involves a certain amount of guesswork, and I would imagine that in most family trees going back more than a few generations there are people whose actual parents are not who we think they were and so everything earlier than that point is completely wrong.
These days we have compulsory registration of births, much less of a stigma about children being born "out of wedlock" so less incentive for people to lie, and of course DNA testing.