One letter came out wrong

Open the Letters room. It shows every letter exactly as it will appear in the font, which is the fastest way to see what actually happened.

Then write the wrong one again. Redoing a letter.

The usual causes

The wrong case was written. A lowercase a written where a capital A was asked for is the most common one, and it hides well — you only notice when a sentence starts with a small letter. Writing the letter we asked for.

A stray mark got included. A rested hand, a slipped pen, or the start of the next stroke. The letter looks right with something extra attached.

A stroke went missing. A t without its cross, an i without its dot — usually because the separate stroke was read as its own mark. When we're not sure about a letter.

It sits off the line. The shape is right but the letter floats or sinks — Letters sit too high or too low.

If the Letters room looks right but your font looks wrong

Then the font is older than the fix. A font file is a finished thing and does not update itself. Make the font again and install the new one — and check the document is using the new version rather than the old one, since both stay installed. I installed it but an app can't see it.

If it keeps coming out wrong

Write it a little more deliberately separated from its neighbouring strokes, without slowing down enough to change your hand. If a particular letter fails repeatedly, tell us which one and what it does — that is worth us knowing.