Hi Hans,

I was going to report a similar issue with \placeformula and floats in general, as in the example below. 
However the trick suggested by Wolfgang does not solve all the problems and in fact in my real size document it triggers an error message saying:

	\strc_formulas_flush_number ...\math_native_reqno 
                                                  {\box \b_strc_formulas_num...
	\strc_formulas_stop_formula ...mulas_flush_number 
                                                  \dostarttagged \t!formulac…

but it is very difficult to set up a minimal example showing this.

In any case here is an example showing the problem with \placeformula and floats, with its PDF attached.

Best regards: OK

% begin bug-placeformula-float.tex

\definefloat[intermezzo]
\setupfloat[intermezzo]
	[leftmargindistance=-12mm,
	rightmargindistance=-12mm,
	default=outer]

\setupcaption[intermezzo][location=none]

\defineframedtext[explication]
	[width=6cm,
	bodyfont=small,
	corner=round,
	frame=on,
	framecolor=darkred,
	rulethickness=1pt,
	background=color, %screen
	backgroundcolor=lightgray] %backgroundscreen=.9
	
\definestartstop[expli]
	[before={\startplaceintermezzo\startexplication},
	after={\stopexplication\stopplaceintermezzo}]
	
%\unprotected\c_strc_formulas_mode\zerocount %% <--- This solves the problem here
%% but not in other instances
\starttext
\startexpli
\input knuth.tex
\stopexpli
\input ward.tex
On dit que la loi de composition interne $\oplus$ est {\bf associative} si on a 
\startformula
\forall\, x,y,z \in G, \qquad x\oplus (y \oplus z) = (x\oplus y) \oplus z.
\stopformula
\input ward.tex

\input tufte.tex
\stoptext
% end bug-placeformula-float.tex