A draped landcover polygon rendered hundreds of kilometres offshore — but only on WebGPU. Every vector slice of one source shares one Material uniform pool, and each slice resets the pool cursor to 0, so a later slice's tile i overwrote an earlier slice's tile-i buffer. WebGPU's queue.writeBuffer defers to submit (last-writer-wins), so every draw read the final bytes; WebGL2's immediate bufferSubData never showed it — and the differential test was structurally blind, because both frames aliased identically.
Half the globe rendered as background again — same symptom as the drape-suppression bug, opposite root cause. This time the trans-antimeridian tiles were never selected: the overzoom fast-path unprojects the viewport corners to a lon/lat box, but on a small globe disc the corners miss the sphere, the box collapses to the sub-camera point, and it emits the single tile column under the camera — which a contiguous longitude range can never wrap across the dateline. Told apart from the draw bug by seam location and persistence.
A jq command over a saved MCP result printed nothing and exited 0 — no data, no error, no failure. Every one of those was manufactured by the caller: a wrong schema emptied stdout, a defensive 2>/dev/null ate the one diagnostic line, and | head laundered jq exit code 5 into 0. A Unix pipeline has three signal channels; habits acquired as defenses can close all three at once.
Labels decayed 6980 → 2138 → 76 → 0 pixels across four probe runs — in commit order, looking exactly like a regression gradient. Every HTTP fetch returned 200. The frame trace said all 46 labels placed. The real bug: an upload staging buffer invisible to the render-on-demand idle predicate, a deadlock that fossilized the map mid-load on any machine. One line made the buffer visible; labels went 0 → 7,081 pixels with zero interaction.
One debugging day, seven instrument failures: a probe that double-counted its own bookkeeping into '93 duplicate requests', a WebGPU canvas that reads back as all zeros, a parity test that passed by comparing two empty frames, a screenshot taken 7.5 seconds too early, and three more. Each with the tell that gave it away and the cross-check that caught it — because the same day also proved that one of those 'flakes' was hiding a real bug.
A static map view produced 10+ MB/s bursts and a warm phone. The render loop was innocent, our first probe fabricated a '93 duplicate requests' finding, and the real villain was the availability feature that keeps fast-pan from blanking — politely fetching an 85-tile, 33 MB world pyramid on every load and pinning it against eviction forever. Same view after the fix: 35.33 MB → 6.27 MB.
Half the globe rendered as pure background, cut exactly at the 0-meridian z1 tile boundary. The fallback ancestors for the missing half were computed, CPU-cached, and force-uploaded to the GPU — then drawn nowhere, because a drape suppression flag was scoped to the renderer instance instead of one dispatch. Verified by making the network slow on purpose, after the first pixel readback lied.
A row of country labels floated in the sky above the globe's horizon. Round 1 — an angular margin, provably safe at every zoom — died because floaters and healthy labels interleave in angular depth (Nigeria 0.332 floats, Kenya 0.329 doesn't). Round 2's screen-space limb fixed Chad and missed two-line Burkina Faso by exactly one line of text. The gate only held once it moved to where the quad height actually exists.
A displaced vector layer, a red/blue checkerboard ocean, a 16-gon planet, and intermittent flicker — four user-visible globe defects on WebGL2, and not one root cause in shared code. Every camera rendered correctly on WebGPU. On why a hand-maintained backend twin is scaffolding that must die, and the 20.7 km test witness that keeps its seam from drifting while it lives.
A missing hemisphere, labels floating off the globe, and tile seams at z6.73 were filed as three engine bugs. All three were one unsynced camera field in the demo harness — and after the fix, review found the identical write one harness file over.
A bug report for one-way arrows drifting off the road came with a diagnosis and a one-line fix. The diagnosis was refuted by reading the anchor code, and the proposed fix was a revert of a change the repo had already tried and backed out. The real fix was neither.
A shader drew correctly on one GPU and returned the clear colour on another — not precision, but layout:auto deleting a binding it declared but never read. When an interface is derived from usage, an unused declaration is a mismatch only the strict backend reveals.
A recursive no-op Proxy let a WebGPU-typed engine boot on WebGL2 by making every native GPU call return a harmless dummy. It also silently swallowed every place a fence was missing. Replacing it with a fail-loud stub — and what the stub screamed about.
Incident report: the fp64 demo alternated between correct and f32-collapsed frames under byte-identical inputs. Inferred cause: driver pipeline re-optimization specializing on uniform values. Fixes: render-on-demand and a texel-fetched guard.
Three real bugs from making WebGL2 match WebGPU on a 117-layer basemap: a dedup map nobody cleared, a depth jitter that out-voted painter's order, and a vertically mirrored composite hidden by a symmetric test fixture.
