Mistakes appear in both the English and Welsh versions of the document.
The Welsh word for general - "cyffredinol" - is spelt wrongly on the cover.
Ms Williams said the party had proof read its work, but that some "typos" had "sneaked through".
One mistake in the manifesto's chapter on education says the party will order a review of "how our schools or held accountable".
In another error, it says schools in Wales receive less money than their English counterparts because "there as is spending gap of over £600 per pupil".
While the English manifesto carries the phrase "Wales can do better" on the cover and every other page, the Welsh version inserts an extraneous "yn" in the sentence "Gall Cymru yn wneud yn well".Continue reading the main story
I hope somebody has proof read the Welsh Liberal Democrat manifesto”End Quote Peter Black Lib Dem candidate, blogging before the mistakes were revealed The word "cyffredinol", meaning general in the phrase general election, appears as "cyffreddinol" on the cover of the manifesto.
There are a series of errors on the Welsh-language contents page, including incorrect spellings of the Welsh words for environment, sports and politics.
The leaders' foreword contains the mangled sentence: "We have people up and down Wales are ambitious to go green, but a Government that subsidising pollution and ignoring the needs of rural areas."
The mistakes come a day after it emerged Plaid Cymru had wrongly attributed the name of its manifesto, Ambition is Critical, to poet Dylan Thomas.
In his blog on Tuesday night, Lib Dem candidate Peter Black wrote: "I hope somebody has proof read the Welsh Liberal Democrat manifesto, which is being launched tomorrow."
Ms Williams said: "Well, we've proof read and proof read and proof read.
"But it just goes to show, doesn't it, that we really need to do something about literacy and numeracy in our country, and I regret that some of those typos have managed to sneak their way through the system.
"But I think there's a very clear sense in that manifesto about what we want to do transform our nation's fortunes."
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