Where have my characters gone?

You've typed some apparently reasonable text and processed it, but the result contains no sign of some of the characters you typed. A likely reason is that the font you selected just doesn't have a representation for the character in question.

For example, if I type “that will be £44.00” into an ordinary (La)TeX document, or if I select the font rsfs10 (which contains uppercase letters only) and type pretty much anything, the £ sign, or any lowercase letters or digits will not appear in the output. There's no actual error message, either: you have to read the log file, where you'll find cryptic little messages like

snippet.latex

Missing character: There is no ^^a3 in font cmr10!
Missing character: There is no 3 in font rsfs10!
``` 
(the former demonstrating my TeX's unwillingness to deal in characters
which have the eighth bit set, while the `rsfs10` example shows that
TeX will log the actual character in error, if it thinks it's
possible).
 
Somewhat more understandable are the diagnostics you may get from
[dvips](https://ctan.org/pkg/dvips) when using the OT1 and T1 versions of
fonts that were supplied in Adobe standard encoding:
dvips: Warning: missing glyph `Delta'

The process that generates the metrics for using the fonts generates an instruction to dvips to produce these diagnostics, so that their non-appearance in the printed output is less surprising than it might be. Quite a few glyphs provided in Knuth's text encodings and in the Cork encoding are not available in the Adobe fonts. In these cases, there is a typeset sign of the character: dvips produces a black rectangle of whatever size the concocted font file has specified.