All bugs occur when weave prints changed modules only.

bug 1: Excessive memory usage.
reason: All modules \mark their module numbers on the current page.
In tex.tex modules 880 to 1332 put their marks all on page 25, since
most of these modules are unchanged. The fix is obvious: mark
your module number only when you are going to be printed.

bug 2:  The module number in the header sometime differs from the first
module on the page.
reason: To keep it simple, we denote the four kinds of Modules with letters:
m: unchanged unstarred Module
n: unchanged starred Module
M: changed unstarred Module
N: changed starred Module

The module sequence "n M", leads to the course of actions:
1. n marks its number on page p.
2. n \ejects p.
3. M marks its number on page p+1
4. M tentativly tries a page break. This break is prohibitive expensive.
   M is still at the start of p+1. 
5. Eventually page p+1 is shipped out.
6. The \output routine writes the last mark from the previous page as module
   number in the header. This is the number of n.
The fix is obvious: Don't \eject a page when you are not going to
be printed. This way there is always some text of the previous
module (or of the initial "\vbox to \vsize".) when it marks its
number.  If the break after the mark is performed, M will have put
its mark on the previous page and the heading will show its number.
If the break is not performed, there will be some text some text
of the previous module, that is, M will not print at the start of
the page and the heading shows correctly the number of the previous
module.

bug 3:  A (nonstarred) module starts on a new page, even though there
was enough room left for the whole module on the old page.
The fix of bug 2 also fixes bug 3.

bug 4:  Some starred modules are not listed in the contents file.
This bug is introduced by the fix of bug 2.
This bug is triggered, when the first module of the program is starred but not
changed.
Writing the contents line is delayed until the current page is
shipped out. Because only then the page number is determined. But
the very first page, the dummy page, will never be shipped out.
Because the dummy page is not ejected before n writes its content line, the
write will get lost.
The fix is obious: Maintain a flag "dp" with the meaning: "Writing
to dummy page" and collect the lines in a macro like this:
\ifdp
  \edef\cline{\cline "line for this module"}
\else
  \edef\cline{"line for this module"}\cline
\fi

bug 5: Sometimes the heading shows the wrong part name 
The fix of bug2 introduces bug5.
A starred module defines its part name in the macro \rhead.
The \output-routine then displays \rhead in the heading.
The sequence n1.p1. n2.p2 M3 N4.p3. triggers the bug: M2 is member of part p2,
but the heading for M2 displays p3 instead of p2. That's because N4 puts p3
in \rhead and then \ejects. The \ejected page starts with M3 but the
\output routine includes the \rhead as defined at the time of \output.

The fix is obvious: Store both the part name and the module number in
the mark. And use both components of the mark to fill the header.

bug 6: \input webmac and the say \bye. The \output will \cycle and fill your
disk.
The fix of bug5 triggers this bug.
What happened? The mark (to the dummy page) is defined to contain \rhead
and \def\rhead{\.{WEB} OUTPUT} breaks the execution of the tokenlist in
the mark.

The kludge is obvious: \def\rhead{WEB OUTPUT} saves the day. The
\.-Macro kills the \output routine.

Why and how the \. macro does its devastating deeds, you ask? Don't
know, I answer.

bug 7: weaving tex.web w/o tex.ch will give you wrong part names in the
header, namely the one of the preceding part.
case: N1.p1 M2 N3.p2 will display the part name WEB OUTPUT for N1.p1
and part name p1 for N3.p2. It should show p1 and p2.
The reason is simple: tex.tex defines a private macro \N which assumes
the old \output routine which puts the part name via \rhead into the header.
(See bug5)
The fix is obvious: \NM is invoked only by the private unchanged \N of
pure TeX or mf. This sets a flag, that controls the \output routine to
get the part name from the topmark (not compatible) or from the \rhead.