Most people know the search pattern is interpreted. Fewer realise the replacement is too.
The ampersand expands
In sed, an unescaped & in the replacement is substituted with the entire matched text. Replacing a term with AT&T inserts the match where the ampersand was, producing output that looks like a bizarre corruption rather than an escaping error. Escape it as \&.
Backslashes and group references
\1 refers to a capture group. A replacement containing a literal backslash needs doubling, and a Windows path in a replacement string is almost entirely backslashes.
Delimiter collision
sed 's/old/new/' breaks when either side contains /, which URLs always do. Any character can be the delimiter — s|old|new| — and choosing one absent from both strings is the fix.
Order matters when chaining
Two replacements applied in sequence are not commutative. Replacing a with b and then b with c turns the original a into c. Where that is not intended, replace via a placeholder that cannot appear in the data.
Try it: Find and Replace Text on SeoWolf's Notepad.