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

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 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.